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Published online 1 March 2006
Published in J Environ Qual 35:548-557 (2006)
DOI: 10.2134/jeq2005.0230
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Ecological Risk Assessment

What is Soil Organic Matter Worth?

G. P. Sparlinga,*, D. Wheelerb, E.-T. Veselyc and L. A. Schippera

a Landcare Research, Private Bag 3127, Hamilton, New Zealand
b AgResearch, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand
c Landcare Research, Private Bag 92170, Auckland, New Zealand

* Corresponding author (sparlingg{at}landcareresearch.co.nz)

Currency rate: The average bank exchange rate 2000–2004 for the New Zealand dollar was Euro 0.501 (high 0.557, low 0.438), United States dollar 0.518 (high 0.727, low 0.389), and Australian dollar 0.850 (high 0.948, low 0.742). Source: OANDA FX History 2005 (http://www.oanda.com/convert/fxhistory; verified 13 Dec. 2005).

Received for publication June 8, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The conservation and restoration of soil organic matter are often advocated because of the generally beneficial effects on soil attributes for plant growth and crop production. More recently, organic matter has become important as a terrestrial sink and store for C and N. We have attempted to derive a monetary value of soil organic matter for crop production and storage functions in three contrasting New Zealand soil orders (Gley, Melanic, and Granular Soils). Soil chemical and physical characteristics of real-life examples of three pairs of matched soils with low organic matter contents (after long-term continuous cropping for vegetables or maize) or high organic matter content (continuous pasture) were used as input data for a pasture (grass–clover) production model. The differences in pasture dry matter yields (non-irrigated) were calculated for three climate scenarios (wet, dry, and average years) and the yields converted to an equivalent weight and financial value of milk solids. We also estimated the hypothetical value of the C and N sequestered during the recovery phase of the low organic matter content soils assuming trading with C and N credits. For all three soil orders, and for the three climate scenarios, pasture dry matter yields were decreased in the soils with lower organic matter contents. The extra organic matter in the high C soils was estimated to be worth NZ$27 to NZ$150 ha–1 yr–1 in terms of increased milk solids production. The decreased yields from the previously cropped soils were predicted to persist for 36 to 125 yr, but with declining effect as organic matter gradually recovered, giving an accumulated loss in pastoral production worth around NZ$518 to NZ$1239 ha–1. This was 42 to 73 times lower than the hypothetical value of the organic matter as a sequestering agent for C and N, which varied between NZ$22 963 to NZ$90 849 depending on the soil, region, discount rates, and values used for carbon and nitrogen credits.

Abbreviations: AEC, anion exchange capacity • DBD, dry bulk density • QT, QuickTest


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SOIL ORGANIC MATTER may be lost rapidly under intensive cropping systems because of the increased rates of organic carbon (C) mineralization following tillage, in addition to erosion losses and decreased organic matter returns through crop removal (Paustian et al., 1998; Bolinder et al., 1999; Gregorich et al., 2001). Organic C losses in New Zealand soils under long-term cropping can be high, with 70 or 80% of organic matter in surface soil lost over 40 to 50 yr (Haynes and Tregurtha, 1999; Sparling et al., 2000; Shepherd et al., 2001).

Many soil scientists advocate the conservation of soil organic matter because of the modifying effects organic matter has on soil properties. These effects are usually beneficial for plant growth and production. Effects include greater water retention and availability, higher cation exchange capacity, the ability to retain nutrients within the root zone, greater buffering capacity against pH change, the ability to chelate and complex ions, contribute to soil structure and form stable aggregates, and sustain biological activity and biodiversity by providing food and habitat for soil animals and microorganisms (Allison, 1973; Gregorich and Carter, 1997; Degens et al., 2000).

While there are many examples that higher organic matter contents can benefit plant productivity (Entry et al., 1996; Monreal et al., 1997; Francis et al., 2001), under intensively fertilized and irrigated arable farming with soil stabilizers, there may be no detectable benefit of high versus low organic matter content because of the compensatory use of improved crop varieties, fertilizer, pesticides, tillage, erosion control, and irrigation (Sojka and Upchurch, 1999; Letey et al., 2003). Further, lower organic matter contents may have unexpected benefits, such as the ability to decrease pesticide use because of decreased soil sorption (Sojka and Upchurch, 1999). However, the need for such extensive management interventions in intensive arable farming does suggest that the presence of soil organic matter contributes to production targets.

Soil organic matter also influences environmental processes at a global scale. Topsoils are a huge terrestrial reservoir of C, which has a modifying effect on carbon dioxide concentrations in the atmosphere and can thus influence climate warming (Paustian et al., 1997; Lal, 2001). In New Zealand, the soils under indigenous forest and grassland typically contain 44 to 268 Mg C ha–1 in the top 1 m of soil (Tate et al., 1995), and managed pastures and some cropping systems contain 70 to 130 Mg ha–1 of organic C in the top 0.2 m of soil (Percival et al., 2000; Parfitt et al., 2002; Sparling et al., 2003a).

Soil organic matter is also a very large store of nitrogen (N), with over 90% of the N in soils in organic forms (Batjes, 1996). The N content of the organic matter in New Zealand soils has increased since clearances of the indigenous broadleaf-podocarp forests and the introduction of European grass–clover pastures in the mid 1800s (Jackman, 1964; McIntosh et al., 1997; Sparling and Schipper, 2002). New Zealand pasture soils typically contain 1 to 10 Mg N ha–1 in the top 0.5 m of soil (Schipper et al., 2004) and 2.3 to 5.9 Mg N ha–1 in the top 0.1 m (Sparling and Schipper, 2004). The mineralization of organic N is an important source of N for pasture grasses, and until the recent increase in the use of N fertilizers, was usually the major source of N used in New Zealand grass–clover pastoral systems. The amount of N fixed by clovers has been estimated to be 100 to 300 kg ha–1 yr–1 (Ledgard and Steele, 1992).

Our objective was to assess the relative value of organic matter for plant production and for C and N sequestration, and to define the financial implications of a decline on soil organic matter. Our approach was to calculate the value in restoring organic matter in a depleted soil. The value of soil organic matter to production was estimated from the value of dairy milk solids based on a computer simulation of pasture dry matter yield and organic matter accumulation. We also estimated the hypothetical financial gain associated with organic matter recovery if C credits were issued for organic C sequestration, and N credits for N sequestration. The simulations and estimates were completed for three real-life contrasting soils and climate regimes in New Zealand.

Both approaches provide only partial estimates of the economic value of soil organic matter. The total economic value of organic matter includes other indirect "ecosystem service" benefits that a soil with high organic matter content provides (Patterson and Cole, 1998). These include improved water storage and release, erosion control, sources of biodiversity, retention of contaminants, and faster degradation of wastes (Patterson and Cole, 1998; Pretty et al., 2001); however, they were beyond the scope of our present study.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Sites and Soils
Examples of paired land uses under long-term pasture or continuous cropping (vegetable or maize cultivation) in three contrasting regions were used to establish the differences in soil organic C and related soil characteristics (Table 1). The soils with their location, cropping history, soil name, New Zealand Soil classification (Hewitt, 1998), and the nearest USDA Soil Taxonomy Great Group equivalent (Soil Survey Staff, 1992) were at: (i) Pukekohe (Patumahoe soil, Granular Order, Great Group Haplohumult), (ii) Palmerston North (Kairanga soil, Gley Order, Great Group Endoaquept); and (iii) Oamaru (Waiareka soil, Melanic Order, Great Group Haplustol). Continuous cropping is a comparatively rare and localized land use in New Zealand. Those particular soils and sites were chosen because they are used for growing vegetables or maize, and we had soil data from previous work on organic matter depletion under those long-term cropping regimes (Table 1), plus soil data on matched long-term sites under permanent pasture (Sparling et al., 2003a). For each of the three soils we simulated what effect those differences would have on dry matter yield for grass (Lolium perenne L.)–clover (Trifolium repens L.) pasture and pasture N and P content, in average, dry, and wet years.


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  		Table 1. Location, soil, and land use history of the paired sites at Pukekohe, Palmerston North, and Oamaru, New Zealand, used to simulate effects of low organic matter and associated characteristics on pasture productivity.

 
Pasture Production and Organic Matter Models
Pasture production was estimated using the AgResearch Dynamic Nutrient Model, based on those of Woodward (1998, 2001), and incorporating modifications from Johnson et al. (1989). The main adaptations to pasture production were the inclusion of stolon growth (Hay, 1983; Brock et al., 2000) and nitrogen fixation (Wu and McGechan, 1999) for white clover (Trifolium repens L.), and the use of a nutrient uptake model based on McCaskill and Blair (1988). The responsiveness of pasture species to soil nutrient concentrations was based on the relationships between relative yield and plant nutrient concentrations. The main drivers of pasture production were daily metrological data (rain and minimum and maximum temperature), soil moisture availability, soil data for each soil layer, and initial dry matter inputs and nutrient availability. Initial simulations indicated problems in estimating yield when there was a change in grass species (from Lolium spp. to Paspalum spp., and vice versa) or when species regrowth was required following drought. The model was changed to allow recruitment from seedlings, and to factor leaf age into the senescence model. The model was then recalibrated (Wheeler, 1998) against 4-yr data on pasture composition (total pasture mass, grass and clover content, plant nutrient concentrations, and clover N fixation measured throughout the year) from a site near Te Kuiti (location: N024.648.735) on a Mangatea soil Brown Soil Order. The responses were then fixed across the three sites in this study.

The pasture production model estimates plant nutrient supply from soil organic matter pools, but is not an organic matter turnover model. We had earlier estimated C turnover in the same three soils using the CENTURY model (Sparling et al., 2003a). The outputs on C accumulation during the pasture phase of that study were used in this paper.

Climate Data
For each site, a meteorological station was selected from the National Institute of Water and Atmospheric Research (2004) climate database that had continuous daily rainfall data over at least 16 yr (Table 2). Rainfall, minimum and maximum temperature, and sunshine hours (or radiation and wind speed) were extracted for the average year annual rainfall (closest to annual average rainfall over time period), driest year (lowest annual rainfall), and wettest year (highest annual rainfall) for each site.


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  		Table 2. Climate station, location, and rainfall data used to simulate effects of low soil C contents on subsequent pasture productivity in dry, average, or wet years.

 
Initial Setup Parameters
The simulations were set up as a mowing trial with monthly harvests and 65% return of clippings. A mowing trial was selected because that was the basis of the calibration trials, and the approach simplified the contribution from recycled nutrients. The pasture was assumed to have a high density, with moderately prostrate clover content. A variable water table between 1 and 4 m and maximum rooting depth of 1 m were used. We specified an initial litter content of 2000 kg dry matter ha–1 and moderate earthworm activity. No additional fertilizer was added to maximize the chance of seeing any effects from differences in organic matter contents.

Soil Parameters
Soil parameters used to set up the high C and low C scenarios are shown in Tables 3, 4, 5, and 6. Data on soil characteristics for the 0- to 20-cm depth for the Patumahoe soil were obtained from Sparling et al. (1998), for the Kairanga soil from Sparling et al. (1992), and for the Waiareka soil from Sparling et al. (2000). Data for soil characteristics below 20 cm (the plow layer) were obtained from the New Zealand National Soils Database and were assumed to be the same for both high and low C scenarios. Land use does alter soil chemical characteristics at depths below 20 cm (McIntosh et al., 1997), but the changes are relatively minor compared to those occurring in the surface soil. For the Kairanga soil, A. Carran (personal communication, AgResearch, Palmerston North) supplied Olsen P and QuickTest (QT) S data from a pasture trial on the Kairanga silt loam. For the Patumahoe soil, there was no QT S data and the values from the Kairanga soil were used. This may have underestimated the contribution of QT S to plant uptake as volcanic soils can accumulate S in the lower horizons.


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  		Table 3. Initial ammonium, nitrate, and organic matter quality characteristics for each depth used to set up pasture production simulations at all three sites.

 

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  		Table 4. Soil parameters used to set up the pasture and cropped soil simulations for Patumahoe soil.

 

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  		Table 5. Soil parameters used to set up the pasture and cropped soil simulations for Kairanga soil.

 

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  		Table 6. Soil parameters used to set up the pasture and cropped soil simulations for Waiareka soil.

 
All sites were set up with the same initial soil nitrate and ammonium concentrations, and the same proportional organic C distribution between pools (Table 3). The C pools were similar to those described by McCaskill and Blair (1988). Soil nitrate (NO3–) and ammonium (NH4+) were estimated values from previous simulations; these initial values are not critical as the pools are transformed rapidly. Organic C distribution was based on some common values obtained for previous simulations for pasture soils and was kept constant to remove any confounding effects that organic C quality, rather than quantity, may have had on yield and nutrient content, and because we had no data to justify alternative distributions.

The pasture levels for QT S, Olsen P, anion exchange capacity (AEC), clay, and silt contents were the same for each simulation within a site (Tables 4Go–6). For these parameters, there were either no data on differences between cropping and pastoral soils, or the data obtained indicated that any differences were small and would have no effect on the output. For QT S and Olsen P, pastoral values were used so that the affect of reducing C levels was not confounded with high levels of inorganic nutrients associated with the previously cropped soils. The other soil data required was total soil C, N, P, organic P and S, dry bulk density (DBD), water content at 5 kPa (field capacity) and 1500 kPa (wilting point), and target C to N, C to P, and C to S ratios. Each simulation was started with the soil at field capacity, and hence at the same water potential.

The pasture production model uses fixed depths to determine water status, nutrients, and root growth. However, some soil parameters at lower depths had only been determined on a horizon basis. To obtain the values for the fixed depths used in the model, a relationship was fitted to the mid-depth of each horizon and values for the fixed depths calculated by interpolation.

Initial examination of the soil data indicated that C to N, C to organic P, and C to S ratios could vary between cropping and pastoral soils. The model was modified to allow these values to return to preset target ratios over a 5-yr period. The time period was based on the results from Sparling et al. (2003b) on rates of organic matter recovery after erosion. Nutrients were removed from the plant available pool and added to the organic matter pool only if the nutrient concentrations were above a minimum value in the plant available pool after all other processes (plant uptake, leaching, denitrification, "normal" immobilization) had occurred. If immobilization did not occur in a given day, then there was no compensatory increase in the rate in subsequent days. For P, the model was modified to include organic P as an input, otherwise organic P was estimated from total P and total N.

Productivity Related Value of Organic Matter
Outputs from the pasture production model used in this study were clover dry matter (daily basis), grass dry matter, and their N and P contents. We elected to convert the total annual pasture production to milk solids equivalent. Dairy farming is a preferred use for good quality pastures in New Zealand (around 40% of cattle numbers) as it provides a superior economic return to farmers. Other land use and production scenarios are possible, but for this study we restricted ourselves to the dairy farming scenario, as this was likely to generate the greatest economic return. The annual pasture dry matter yield under the various scenarios was converted to milk solids using the factor 1000 kg dry matter = 14.3 kg dairy milk solids (Penno et al., 1996). No adjustment was made for seasonal differences, clover content, or N and P concentrations. The yield of dairy milk solids was then converted to a monetary value using a national milk solids price of NZ$3.14, obtained by averaging dairy company payouts to farmers between 1987 and 1996 and adjusting for inflation (Ministry of Agriculture and Forestry, 2004). This price to New Zealand dairy farmers has stayed reasonably stable despite changes in international markets and exchange rates, and ranged from a high of NZ$4.03 in 1996 to a low of NZ$2.70 in 1991. The difference in monetary value of the yields from the low and high organic matter content soils was used to estimate the productivity benefit of soil organic matter.

Environmental Value of Organic Matter
The C sequestered in organic matter (Mg ha–1 to 1 m depth) was estimated and converted to an equivalent mass in C credits. Sparling et al. (2003a), using the CENTURY model, estimated that after 25 yr of continuous pasture on a previously depleted soil, the accumulation of C in the top 20 cm would be 21, 9, and 15 Mg ha–1 for Granular, Gley, and Melanic soils, respectively. That calculation allowed for recovery to 80% of the original value (Parshotam and Hewitt, 1995; Sparling et al., 2003a). Assuming the same rates of recovery apply throughout the 1-m soil profile, we would expect the beneficial effects of the organic matter recovery to persist (but with a declining effect relative to permanent pasture) for up to 36 yr for the Granular Soil (Pukekohe), 90 yr for the Gley Soil (Palmerston North), and 125 yr for the Melanic Soil (Oamaru).

Because the New Zealand market in C credits is small and may be unrepresentative, we used the value of C credits in the European and UK carbon market in November 2003 which were trading at NZ$6 to NZ$17 Mg–1 (Point Carbon, 2004). The current New Zealand trading value of NZ$11 to NZ$19 used by the EBEX21 scheme (EBEX21, 2005) overlapped that range.

The amount of sequestered N was calculated assuming that the recovery curve for N follows the same pattern as for C, and had a C to N ratio of 11:1 (Sparling and Schipper, 2004).The value of the N sequestration service was estimated assuming the existence of nutrient trading schemes. Trading schemes are operational in the United States and Australia (Sessions and Leifman, 1999; Kraemer et al., 2003) and have been explored in New Zealand as a potential economic instrument for managing water quality (MacDonald et al., 2004). We used the price set by the Connecticut Nitrogen Credit Exchange Program for 2003, namely US$2.14 per equalized pound of N (McCarthy, 2004). This rate was transformed using the mid-point rate at 31 Jan. 2003 of US$0.5469 = NZ$1 (Reserve Bank of New Zealand, 2003) and a 1 to 1 equalization factor resulting in NZ$8.70 per kg of N. This point estimate is a real life example of a rate at which trading has occurred; however the cost of removing 1 kg of N varies widely. Sweeney (2004), estimating the cost of removing N through a range of nonpoint-source best management practices, identified a range between NZ$1.4 and NZ$216 (US$0.73 and US$112) per kg N. Melbourne Water, for its offset scheme, calculated the cost of removing N through large regional wetlands in Australia would be NZ$940 per kg of N for the 2005–2008 period, which could increase to more than NZ$2350 per kg of N in the 2008–2011 pricing period (Francey and Chesterfield, 2005).

Net Present Value
Due to the extended timeframe in which the effects of the organic matter recovery are expected to persist, net present value calculations were performed. The net present value is the difference between the present value of future revenue and the present value of future costs for an activity over a given period (Australian National Audit Office, 2001). The use of discounting enables the different cost and benefit flows to be converted into a single net present value for decision-making (Young, 2002).

The net present value of organic matter recovery in these New Zealand soils was calculated by summing the costs associated with lower productivity and the benefits associated with C and N sequestration over their respective recovery period, namely 36 yr for the Granular Soil (Pukekohe), 90 yr for the Gley Soil (Palmerston North), and 125 yr for the Melanic Soil (Oamaru). To allow for the sensitivity of the net present value calculations to the choice of the discount rate, two different rates, 10 and 3.5%, were employed. The New Zealand Treasury uses a 10% real rate whenever there is no other agreed sector discount rate and has calculated the social rate of time preference to be 3.5% (Young, 2002).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Climate and Soil Characteristics
Pukekohe was the wettest site and Oamaru the driest (Table 2). The Kairanga pasture soil at Palmerston North had the greatest initial organic matter content. All three soils had lost organic matter C and N during the cropping phase, but total C declined proportionally more than total N (Tables 4Go–6). Declines in total P were less marked than C or N. The field capacity of the cropped soil was frequently less than the pasture soil.

Pasture Yields on Formerly Cropped Soils
Annual pasture dry matter yields on the formerly cropped low C soils were always less than the high C soils (Table 7). At Pukekohe the "average" year gave the greatest dry matter return, but at Palmerston North there was comparatively little difference in dry matter yield under the different climate scenarios. At the Oamaru site the "dry" climate scenario gave a much lower annual pasture dry matter yield than the average or wet years. Decreases in pasture dry matter on the low C soils ranged from 2127 to 3340 kg ha–1 year–1 at Pukekohe, to 1169 to 1674 kg ha–1 yr–1 at Palmerston North, and 592 to 1462 kg ha–1 year–1 at Oamaru.


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  		Table 7. Total annual pasture dry matter yield for an average, dry, and wet year, of previously cropped sites and pasture sites in Pukekohe, Palmerston North, and Oamaru, the decreased yield attributable to cropping, the equivalent loss in milk solid production, and the net present value of the decrease in milk solids for the recovery periods of the organic matter content applying 3.5 and 10% discount rates. Figures in parentheses show proportional decrease relative to high C pasture soil.

 
Allowing Organic Matter to Recover
The lower pasture yield from soils with less organic matter resulted in an annual decrease of 8.5 to 47.7 kg of milk solids ha–1, with a monetary value of NZ$27 to NZ$150 ha–1 (Table 7). For the recovery periods of 36, 90, and 125 yr, the cumulative loss per ha due to decreased productivity at Pukekohe was estimated to be NZ$1239 at a discount factor of 3.5% or NZ$772 at a discount of 10%. Equivalent values for Palmerston North were NZ$1092 and NZ$518 and at Oamaru were NZ$1237 and NZ$546.

Total C losses from the 0- to 1-m depths of the three profiles under cropping amounted to 27 to 73 Mg ha–1 representing a 14 to 28% decline in total C content in the top 1 m relative to pastures (Table 8). If the organic matter content is allowed to recover and the associated carbon credits traded, then the environmental service would be worth NZ$5678 to NZ$16 001 (3539–9974 at 10% discount) at Pukekohe, NZ$4534 to NZ$12 839 (2151–6092 at 10% discount) at Palmerston North, and NZ$3665 to NZ$10 536 (1619–4655 at 10% discount) at Oamaru for the recovery periods indicated above.


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  		Table 8. Total organic C content in the top 1 m of three New Zealand soils, the decrease due to the previous cropping regime, and the net present value of C credits in NZ$ if the C increase during the recovery of the low organic matter content had been traded. Figures in parentheses show the proportional decrease relative to a high C pasture soil.

 
Total N losses from the low organic matter soils were least at Oamaru (1.1 Mg ha–1) and greatest at Pukekohe (6.6 Mg ha–1), representing a decline of 8 to 41% in the top 1 m relative to the high organic matter soils (Table 9). If the organic source of N is not replaced by fertilizer but let to re-accumulate naturally, the N sequestration service per hectare would be worth NZ$74 845 (46 651 at 10% discount) at Pukekohe, NZ$59 764 (28 356 at 10% discount) at Palmerston North, and NZ$48 306 (21 344 at 10% discount) at Oamaru for the same recovery periods used for the calculation of C credits (Table 9).


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  		Table 9. Total organic N content of three New Zealand soils (0- to 1-m depth), the decrease due to the previous cropping regime, and the net present value of N credits in NZ$ if the N increase during the recovery of the low organic matter content had been traded. Figures in parentheses show the proportional decrease relative to a high C pasture soil.

 
For the three soil orders, the net present value of accumulating organic matter over their various recovery periods in the three soils was calculated as the cost associated with lower productivity, offset by the benefits of C and N sequestration. This benefit was between NZ$50 734 and NZ$89 610 if the 3.5% discount rate is applied, and between NZ$22 417 and NZ$55 853 if the 10% discount rate is applied (Table 10). The benefits to the environment through C and N storage were 42 to 73 times greater than benefit through greater pasture production.


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  		Table 10. Accumulated value of soil organic matter recovery of soils at Pukekohe, Palmerston North, and Oamaru, estimated from the loss of production and the environmental benefit from increased storage of C and N during the recovery phase, discounted at 3.5 and 10%.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Model simulations showed that prolonged cropping leading to lower organic matter C and N, and associated changes in soil chemical and physical condition, resulted in decreased dry matter production at all three sites; the decreased production was more marked during the wet year scenarios. We had anticipated organic matter would have a greater benefit during the dry year scenarios, as there is normally a soil moisture deficit during the summer months, and greater moisture retention might be expected to improve yields. A possible explanation is that it is the weather patterns during critical periods of pasture growth that are important in determining total annual production, rather than the total annual rainfall. We modeled only the actual driest years from our climate database and did not attempt different scenarios with the same total rainfall but with different distribution.

The lower pasture production from the previously cropped soils was estimated to cost NZ$27 to NZ$151 ha–1 yr–1. We intentionally did not add additional fertilizer to the scenarios to maximize any benefits from organic matter nutrient supply, but the nutrient supply effect on production was comparatively small. Within New Zealand, trials in which phosphate fertilizers were withheld resulted in reductions in pasture yield of up to 15% over a 7-yr period, depending on the initial soil P status (Roberts and Thomson, 1988; Lambert et al., 1990; Rowarth and Gillingham, 1990). This result infers that nutrient deficiencies due to lower supply from organic matter mineralization could probably be readily corrected by modest additions of inorganic fertilizer. If so, then the production losses would not be as great as predicted by our model. Several groups particularly in the United States (Sojka and Upchurch, 1999; Letey et al., 2003) have argued that organic matter is of limited benefit for production on intensively managed croplands that receive fertilizers and irrigation. Our simulations support their arguments in terms of production from rain-fed pastures in New Zealand. In contrast, the benefit of soil organic matter for environmental protection far outweighed (by 40 to 70 times) the benefits to production, even though we only considered C and N sequestration.

Our scenarios assumed that the quality of the organic matter was similar in the low and high C soils. However, organic matter fractions, particularly labile organic C and N, are reported to be greater under pasture than cropped soils (Haynes, 2000) and in no-till rather than tilled soils (Campbell et al., 1999). A change in organic matter quality, especially a shift to pools with slower turnover rates in the low C soils, could result in less mineralization and decreased nutrient supply. We had no data on organic matter quality for our three soils, but if such changes did occur, then the reductions in yield on sites with low quality organic matter would be greater than modeled here.

We showed organic matter also had considerable value for environmental protection in sequestering C and N to reduce greenhouse gas emissions and to reduce the risk of eutrophication of waters. Pretty et al. (2001) noted the importance of the environmental value of C sequestration. Citing the work of Sala and Parnelo (1997) they calculated the annual value of C sequestration in the U.S. Great Plains to be NZ$390 ha–1 (US$200), four times as great as the net private returns to farmers for meat, wool, and milk, and about half the market value of the land.

Once C-depleted soils start to accumulate organic matter under a more lenient management regime, the negative effect of the lower organic matter for production and the positive effect on ecological benefits will gradually decline as the organic matter content is restored to its original level. Taken over the 36- to 125-yr scenarios, the net present value of the C sequestration benefit was between 2.9 and 13 times as high as the cost associated with lower productivity. The N sequestration benefit was even more valuable, its present value being between 39 and 60 times as high as the present value of the cost associated with lower productivity. Consequently, the introduction of C and N trading schemes could provide financial incentives to land managers for organic matter restoration. For the present, however, the C and N sequestration services are indirect environmental values that have a hypothetical rather than actual financial value. The environmental value of soil organic matter may be very much greater than calculated here, when other direct and indirect effects of organic matter retention, such as erosion control and flood prevention, are also included (Pretty et al., 2001).

We conclude that under the three climate scenarios and three soils examined, the changed soil attributes and lower organic matter content following a cropping regime resulted in a modest decrease in subsequent pasture productivity and associated financial value of product. The net present value of the "lost" production over the recovery periods of 36 to 125 yr were 42 to 73 times lower than the net present value of the environmental services of sequestering C and N to mitigate air and water pollution.


   ACKNOWLEDGMENTS
 
Funding for this project was provided through the New Zealand Foundation for Science and Technology Contract C09X0016.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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