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a Sustainable Land Use Team, HortResearch Ltd., Palmerston North, New Zealand
b Inst. of Soil Science, Univ. of Hannover, Hannover, Germany
* Corresponding author (mdeurer{at}hortresearch.co.nz).
Received for publication September 24, 2007.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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Abbreviations: CHWC, hot-water–extractable soil C • Ct, total soil organic C
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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We focused on soil C management as a specific management practice. Soil C management is defined as "land management practices that maintain or increase soil C" (Kimble et al., 2007). The soil C management cannot be identified as one particular management practice. Several management practices and other variables, such as soil type and climate, influence a soil's C status. We used the total soil organic C (Ct) and the hot-water–extractable soil C (CHWC) as a combined proxy for the soil's C status. The Ct describes the size of the entire soil C pool. The CHWC characterizes the labile C fraction that is well correlated with microbial activities (Ghani et al., 2003).
Soil C is a key property for many environmental soil functions (e.g., filtering excessive plant nutrients or contaminants from water) (Pierzynski et al., 2007). Following the soil quality framework (Karlen et al., 2001, 2003), management practices would be sustainable if key soil functions did not degrade. The relationship between soil functions and soil organic C is often indirect. Soil organic C is correlated with biophysical soil properties, which in turn govern soil functions. For example, higher soil C might lead to higher microbial biomass (Sparling, 1992), which accelerates the degradation of organic contaminants.
The performance of the soil functions usually depends on several soil properties and on the initial and boundary conditions for the specific site. For example, the degradation of organic contaminants in the root zone with high microbial biomass might be prevented by an atmospheric boundary condition that leads to preferential flow. In that case, the contaminants would be rapidly transferred from the soil surface to below the root zone. The microbes in the root zone would not interact with the organic contaminants, and, therefore, degradation is low. Under these conditions, there would be no relation between a variation of microbial biomass in the bulk soil and contaminant decay.
Biophysical properties in topsoils of organic and conventional production systems have been compared (Carpenter-Boggs et al., 2000; Castillo and Joergensen, 2001; Fliessbach et al., 2007; Goh et al., 2001; Gunapala and Scow, 1998; Monokrousos et al., 2006; Werner, 1997). However, the change in values of biophysical properties of integrated, conventional, and organic systems have not been quantitatively attributed to a specific set of management practices. We quantified the impact of soil C management on biophysical soil properties in two apple production systems. Our objectives were (i) the presentation of a framework to quantify the impact of soil C management on selected environmentally relevant biophysical soil properties and (ii) the application of the framework to the topsoils of an organic and integrated orchard system.
In this study, the microbial biomass, basal respiration, respiratory quotient, and dehydrogenase activity were used as microbiological soil characteristics. Microbial biomass and basal respiration were selected as key variables to represent C and nitrogen (N) dynamics in soils (Grant et al., 1993a,b). The sequestration of C and N, the emission of CO2 and N2O, and the degradation of organic contaminants are environmentally relevant soil functions that result from C and N turnover processes. The respiratory quotient combines basal respiration and microbial biomass. It quantifies the efficiency of the C turnover by the soil's microbial biomass. The dehydrogenase activity is a sensitive measure of the microbiological status of a soil (Taylor et al., 2002) and is often used to indicate the impact of land use on the soil's microbiological properties (Parham et al., 2002; Quilchano and Marañón, 2002). It is also well correlated with environmentally relevant soil functions, such as the emission of CO2 and N2O (W
odarczyk et al., 2002).
For describing the soil physical properties, we chose the aggregate stability, the amount of plant-available water, and the conductive pore diameter close to water saturation. Aggregate stability is a key property preventing soil erosion (Le Bissonais and Arrouays, 1997) and loss of phosphorus to surface waters (de Jonge et al., 2004). Higher amounts of plant-available water in the root zone enable a more efficient irrigation management, avoiding the leaching of excessive nutrients below the root zone. We measured the conductive pore diameter near water saturation (Sauer et al., 1989) as an indirect indicator for preferential flow.
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Materials and Methods |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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Framework to Quantify the Impact of Soil Carbon Management on Biophysical Soil Properties
The observation time (
t) for performing the following statistical treatment should be long enough to represent the interaction of the local climate with the biophysical soil property (f). In our case, we suggest a minimum measurement period of 1 yr for the biological soil properties.
Formal Setup of Variables
We compared the values of a biophysical soil property f at location xi and xj over a time interval t. The location xj served as a reference and did not receive all management practices but was in all other respects comparable to xi. This meant, for example, that xi and xj had the same soil type, texture, and climatic conditions and the same initial conditions such as recent land use history.
Calculation of the Impact
The impact of the C management on the biophysical soil property at xi and over the time interval t, f(xi;t), was calculated in five steps (Fig. 2
).
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t) and P(xj;t) were statistically significantly different, then a potential impact of the soil C management on the biophysical soil property f(xi;t) was probable, and we proceeded to the next step.
We tested if the soil biophysical property of the managed soil f(xi;t) and of the reference f(xj;t) were statistically significantly different. Only if this was the case did we assume there was an impact of any management on f(xi;t), and we proceeded to the next step.
The ratio 
of the biophysical soil property measured at xi and xj and averaged over t yielded the overall impact of all management practices on f(xi;t):
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multiplied by 100 denoted the percentage difference (larger = positive value and smaller = negative value) in the biophysical soil property at xi compared with the reference xj. Therefore, is a measure of the impact of the management practices on the biophysical soil property at xi.
We performed a regression of the biophysical soil property f(xi,j;t) (dependent variable) versus the respective management proxy values P(xij;t) (independent variable). This yielded the variance fraction (R2) that could be explained by the proxy. We denoted it by R(f(xi,j;t);P). The proxy had to be a statistically significant variable in the regression.
The correlation between the biophysical soil property and the proxy for the soil C management, R(f(xi,j;t);P) (step 4), was multiplied by the impact of all management practices on the biophysical soil property (step 3). By this we estimated the partial impact I of the soil C management P on the biophysical soil property at xi:
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Reference for the Managed Tree Rows
We used the soil in the alley of each apple orchard as the reference for the managed soil in the tree row (Fig. 1). We selected the top 0.3 m. This was the depth of the plow layer of the previous land use (market gardening). For analysis, we separated the top 0.3 m into three increments (0–0.1, 0.1–0.2, 0.2–0.3 m). This enabled us to estimate the depth of the impact of soil C management on the biophysical soil properties.
Measurement Methods
Measurements for all properties, with the exception of the conductive pore diameters near water saturation, were taken from three depths (0–0.1, 0.1–0.2, and 0.2–0.3 m) at three randomly selected locations of the tree rows and alleys (between the marks of the wheel-tracks) of both orchards.
Total Carbon
The soil samples were analyzed by the Dumas Method for %C using a LECO CNS-2000 Analyzer (Laboratory Equipment Corporation Ltd, Castle Hill, NSW, Australia). The C contents were measured in January and November 2006. The CHWC (Ghani et al., 2003) was measured monthly from April to December 2006.
Microbiological Measurements
The microbiological functioning of the soil was characterized by taking monthly measurements from January to December 2006. We measured microbial biomass according to the method of Höper (2006) and basal respiration following 
hlinger et al. (1996).
Dehydrogenase Activity
We used 5 g of soil to perform a dehydrogenase assay with 2,3,5-triphenyltetra-zolium chloride as a substrate (Chandler and Brookes, 1991). The resulting triphenylformazan concentrations of the extracted solutions were measured with a spectrophotometer at 485 nm (DU-640; Beckman, Krefeld, Germany). Using bulk density measurements, the results were transformed to represent soil layers of a unit area of 1 m2.
Physical Measurements
The physical functioning of the soil was analyzed by measuring water retention (Dane and Hopmans, 2002) and aggregate stabilities (Le Bissonais, 1996) in the laboratory and by measuring water infiltration rates with tension disk infiltrometers in the field (Deurer et al., 2008). From the water retention curves, we estimated the plant-available water content as the amount of water that is retained in the soil between –0.064 and –15 bar. The aggregate stabilities were expressed as a mean weighted aggregate diameter (Le Bissonais, 1996). From the infiltration rates at –80 and –10 mm tension, we derived the equivalent conductive macropore diameter (CMD) (Sauer et al., 1989).
Statistical Methods
The results of each production system (organic, integrated) were analyzed with a two-way ANOVA with the Genstat 9.1.0.150 software. We selected the treatment (tree row versus alley) as the first and the soil depth (0.0–0.1, 0.1–0.2, or 0.2–0.3 m) as the second factor. The monthly measurements of microbial properties were represented as blocks. There were three randomly selected replicates for each combination of the two factors within each block. We interpreted the differences between averages of properties to be significant if they were larger than their respective least significant differences (P 
0.05).
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Results |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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Physical Functioning of the Soil: Comparison of Tree Row and Alley
The tree row management enhanced the aggregate stability in the organic system and degraded it in the integrated system (Table 3
). The conductive macropore diameter near water saturation was measured only in the first depth. It increased for both systems in the tree row. The tree row management of both systems tended to decrease the plant-available water in the topsoil and increased it in the subsoil. Generally, the differences of physical properties between the tree row and the reference of both systems were largest in the 0- to 0.1-m and 0.2- to 0.3-m depths (Table 3).
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Total and Partial Impact of the Soil Carbon Management on the Physical Functioning of the Soil
The physical properties showed no clear general trend in the tree row of the organic system compared with the reference (Table 5). The impact of the soil C management on the physical properties in both systems was smaller than on the microbial properties (range, –15 to +17%) (Table 5). The conductive pore diameter near water saturation increased by 94 and 45% in the top 0.1 m of the tree row of the organic and integrated systems, respectively. However, we could not attribute this increase to the soil C management. In the integrated orchard, we observed a considerable degradation of the soil structure (aggregate stability) in the 0- to 0.1-m and 0.2- to 0.3-m depths in the tree row. We estimated that C management explained about half of the loss of aggregate stability.
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Discussion |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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We assumed that the tree roots contributed equally to the C status of the soil of the tree row and of the alley. Consequently, the alley soil reference would not be suited to quantify the impact of any tree-specific management (e.g., pruning) on the soil's C status. However, our objective was to quantify the impact of a set of soil C management practices that applied only to the soil of the tree rows (e.g., compost application, irrigation, or herbicide application).
Another reason to select the soil in the alley as the reference was to make the application of our method practically feasible for growers and regulatory agencies. To be meaningful, the reference soil has to meet the following conditions: Condition 1, same soil type; Condition 2, same climatic boundary conditions; and Condition 3, same initial soil C status as the soil of the tree row.
It is improbable to identify a soil neighboring an orchard that is used as a pasture (i.e., suitable as a reference) and simultaneously fulfils all three conditions. The greatest problem is Condition 3. Condition 3 requires not only that the orchard and the neighboring pasture have the same initial soil carbon status but also that it was analyzed and documented at the time.
The soil quality framework offered no guidance as to which soil depths should be selected (Letey et al., 2003). We chose the depth of the plow layer (0.3 m) of the previous land use. For this layer we could assume that the ploughing created the same initial C amounts for the soil under the alley as under the tree row. Other studies comparing the soil quality of organic and integrated apple production systems characterized the 0- to 0.15-m depth (Glover et al., 2000) or the 0- to 0.3-m depth (Goh et al., 2001). An investigation of the quality of soils across New Zealand and under various land uses focused on the top 0.1 m (Sparling and Schipper, 2004).
Soil Carbon Management
In contrast to the unknown contributions of several soil-C–related management practices, our proxy for soil C management (Ct and CHWC) can be directly quantified. By relating our impact analysis to the soil C status, our results can be generalized and should apply to other land uses and sites.
The soil C amount (Ct, CHWC) under the tree row was significantly smaller than under the alley of the integrated orchard. Soil C conservation was not an objective of integrated orchard management. The tree row without pasture received little input of root-biomass C and no input of C via compost. Additionally, the drip irrigation in the tree row led to continuously favorable moisture conditions for C mineralization and might promote the leaching of dissolved organic C. The lack of C conservation, and thus a loss of C over time in the tree rows of integrated production systems such as apple, kiwifruit, and grapes, are rarely considered. By contrast, the use of pasture as understorey vegetation for C conservation is avoided because it competes with the crops for water and nutrients (Tworkoski and Glenn, 2001). Economic incentives such as C credits (Sparling et al., 2006) or market access regulations that reward environmental stewardship similar to the EurepGAP framework might change this in the future.
In the organic orchard, the management conserved soil C. Soil C inputs into the soil are generally higher in organic than in integrated or conventional production systems (Fliessbach et al., 2007; Gunapala and Scow, 1998).
The pasture and regular compost applications in the tree row of the organic system not only conserved Ct but also led to more labile C (as CHWC) in the tree row than in the alley.
Soil Microbial Functioning
The degradation of the soil C status in the tree row of the integrated orchard translated, in general, to a substantial decrease in microbial activities. Most measures of microbial activity decreased in the tree row to 0.2 m depth. In the organic orchard, the decrease in microbial activities in the tree row was small and often not significant. For example, there was no significant difference between the microbial biomass in the tree row compared with the alley.
A correlation of microbial activities and the soil C status is expected (Fliessbach et al., 2007; Ghani et al., 2003; Sparling, 1992). We quantified the correlations between microbial activities and soil C status separately for the integrated and the organic system. The correlation of the C status with the basal respiration was similar and low for both systems. The soil C status could explain 58% of the variability of the dehydrogenase activities in the integrated system and 81% in the organic system. Soil C seemed to be of different importance for the enzyme activities in the two systems. Another explanation could be that the total and CHWC pools in the soil of the integrated and the organic orchards have not only different sizes but also different qualities.
The basal respiration decreased in the soils under the tree row in both systems. We attributed less than half of this decrease to the change in the soil C status. It is difficult to interpret the decrease of a soil property with respect to the environmental sustainability of the soil C management. For example, the decrease of basal respiration would be positive if less basal respiration indicated a reduction in CO2 emissions. Conversely, it would be negative if it indicated a smaller degradation potential for C-rich organic contaminants, such as herbicides.
A quantification of "positive" and "negative" could be achieved by numerically modeling those functions based on the measured basal respiration. In the soil-quality framework, the sum of positive and negative changes of all environmentally relevant functions would quantify sustainability (Karlen et al., 2003). Consequently, a zero net change of soil quality could then indicate the overall environmental sustainability of C management. The scientific merit of such lumped sums is controversial (Letey et al., 2003; Soijka et al., 2003).
In the integrated system, the dehydrogenase activities decreased by about 60% down to 0.3 m depth. The same order of magnitude was found in another study comparing conventional arable systems without soil C management with organic systems (Fliessbach et al., 2007). However, we could now show that only about half of this decrease was due to the degradation of the soil C status. Therefore, other management practices combined are equally important. The respiratory quotient was recommended as a sensitive indicator for the impact of land-use–related change in soil C status on the microbial functioning of soil (Anderson and Domsch, 1990, 1993; Sparling, 1992). However, the considerable change in the soil C status in the integrated orchard did not have any significant impact on the respiratory quotient. We concluded that the C management of the topsoil of the integrated orchard was not sustainable with respect to environmental soil functions that are sensitive to the soil's microbial functioning, whereas that of the organic orchard was sustainable.
Soil Physical Functioning
The degradation of the soil C status in the tree row of the integrated orchard led to a decrease in the aggregate stability in the topsoil. Less C on the same soil usually leads to lower aggregate stability (Le Bissonais and Arrouays, 1997). However, we could attribute only about half of the decrease in the aggregate stability in the tree row of the integrated orchard to the soil C management.
Compared with the alley, the aggregate stability in the tree row of the organic orchard improved in the 0.1- to 0.2-m depth. We attributed about 70% of this effect to the soil's C status. The amount of CHWC was significantly different between the tree row and the reference at this depth. Therefore, the difference might be an effect of the quality rather than the total quantity of the soil C. For example, compost was applied only to the tree row and not to the alley.
The tendency for preferential flow in the top 0.1 m increased in both systems compared with the alley. However, this was not correlated with soil C status. Another study comparing soils in the tree row and the alley in a New Zealand orchard reported higher infiltration rates in the tree row (Goh et al., 2001). The authors attributed the higher infiltration rates to the compaction by vehicles in the alley. In the apple orchards of our study, the vehicle traffic was confined and created clearly visible wheel-tracks. We sampled the alley between the wheel-tracks.
Many studies that evaluated soil quality in organic and conventional or integrated production systems focused only on biochemical soil properties (Anderson, 2003; Bending et al., 2004; Castillo and Joergensen, 2001; Fliessbach et al., 2007; Gil-Sortres et al., 2005; Monokrousos et al., 2006; Ruf et al., 2003; Schloter et al., 2003). In some studies it was argued that the soil's physical parameters can be neglected in diagnosing soil quality because they have little sensitivity for land use change (Filip, 2002; Gil-Sortres et al., 2005).
We argue that the soil's physical properties play a key role in most environmentally relevant soil functions. We found in our study that the soil C status was better correlated with its biochemical than with its physical properties. As an example, we attributed the lack of correlation of the conductive pore diameter near water saturation with C management, for example, to the fact that many other factors also influence this property, which leads to a spatially high variability. Another study found a weak correlation of soil organic C with the tendency for preferential macropore flow (Jarvis et al., 2007). Currently in New Zealand, the only routine measurement for physical soil quality is macroporosity in the top 0.05 to 0.1 m (Sparling et al., 2004). Other physical properties, such as the hydraulic conductivity and the plant-available water, were not considered for reasons of high variability and high cost (Schipper and Sparling, 2000).
In a forthcoming paper, we plan to use the biophysical soil properties to parameterize a numerical model and evaluate the performance of various environmental soil functions. Then, we will compare the performance of the modeled soil functions in the tree row with its reference (alley) and assess the sensitivity of the performance to the underlying biophysical soil properties. From the study presented here, we know how sensitive the biophysical soil properties are to soil C management. Therefore, we will be able to quantify how sustainable the soil C management is with respect to individual soil environmental functions.
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Conclusions |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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For the integrated production system, we recommend the introduction of regular compost applications and the growth of pasture in the tree rows. This could prevent a degradation of the soil's biophysical functioning, and soil quality could be enhanced.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResultsDiscussionConclusionsREFERENCES |
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hlinger, R., T. Beck, B. Heilmann, and F. Beese. 1996. Soil respiration. p. 93–110. In F. Schinner et al. (ed.) Methods in soil biology. Springer-Verlag, Berlin, Germany.odarczyk, T., W. Stepniewski, and M. Brezezi
ska. 2002. Dehydrogenase activity, redox potential, and emissions of C dioxide and nitrous oxide from Cambisols under flooding conditions. Biol. Fertil. Soils
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:200–206.[CrossRef]
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