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University of Wisconsin, Department of Soil Science, 1525 Observatory Drive, Madison, WI 53706
* Corresponding author (lrcooperband{at}facstaff.wisc.edu)
Received for publication January 8, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: PAW, plant-available water • PMR, paper mill residuals • PMRB, paper mill residuals composted with bark • PMRC, paper mill residuals composted without a bulking agent
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Other studies have demonstrated a clear relationship between total soil C and soil aggregation, bulk density, water retention, and hydraulic conductivity (Khaleel et al., 1981). Benbi et al. (1998) demonstrated that amending coarse-textured soils with manure increased organic carbon content and improved saturated hydraulic conductivity, water stable aggregation, and water retention. Martens and Frankenberger (1992) reported an increase in water infiltration rates in soils that were amended with a variety of organic materials. Several studies with biosolids and composted biosolids showed increased soil water retention and aggregate stability in silt loam soils (Epstein, 1975; Epstein et al., 1976; Wei et al., 1985; Lindsay and Logan, 1998).
Several studies have documented the beneficial use of PMR in crop production. Simard et al. (1998) achieved an increase in soil water-holding capacity of silt loam during one year of PMR application, but no effect the following year. Bellamy et al. (1995) demonstrated the short-term benefits of amending agricultural soils with PMR; however, they recognized a need to study the effects of repeated annual additions of PMR on soil properties.
Composting can be an effective strategy to stabilize PMR prior to land application (Valente et al., 1987; Campbell et al., 1995). The composting process biologically stabilizes heterogeneous raw PMR and reduces mass and volume and thus handling and hauling costs. Evanylo and Daniels (1999) reported that potting media containing PMR compost was better able to supply the high nutrient needs of green pepper (Capsicum annuum L. var. annuum) compared with commercial potting medium. In contrast, Campbell et al. (1995) demonstrated that a mixture of PMR composts and slaughterhouse paunch manure was inhibitory to tomato (Lycopersicon esculentum Mill.) growth, but suitable for growing poplars (Populus spp.). Campbell et al. (1991)(1995) highlighted that many studies have examined the feasibility of composting PMR, but information on the effects of PMR composts on field-grown crops is limited.
Studies involving land application of PMR or PMR composts highlight the variability in chemical and physical properties of these materials, and resultant effects on crop growth (Vagstad et al., 2001). Depending on the paper-making process (new versus recycled or de-inking) and the relative proportions of primary and secondary wastewater treatment residuals present, the PMR can have very high (>70) to low (<20) C to N ratios and varying amounts of clay, calcium carbonate, or other mineral fillers. Field experiments with high C to N ratio materials like de-inking sludge have shown adverse effects on crop growth, especially when applied just prior to crop planting (Fierro et al., 1997; Harrison et al., 1996).
Few studies have explored the cumulative effects of annual additions of PMR or PMR composts on soil physical properties, especially with moderate-rate applications of low C to N ratio PMR amendments. The overall objective of our research was to evaluate the short- and intermediate-term effects of annually amending sandy soils with raw PMR, PMR composted alone (PMRC), or PMR composted with bark (PMRB) in a vegetable rotation in Wisconsin's Central Sands region. Specifically, we investigated the effects of PMR, PMRC, and PMRB on soil physical properties such as bulk density and water-holding capacity and related these properties to changes in total soil C. We hypothesized that composted PMR would have longer-lasting effects on soil physical properties compared with raw PMR due to differences in amounts of C applied and decomposition levels.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Amendment Types and Characteristics
The amendments included raw PMR, PMR composted without a bulking agent (PMRC), or PMR composted with bark (PMRB). Mixed hardwood bark is another waste stream from the paper-making process, as pulp logs are debarked prior to pulping. Dark peat was included as a relatively "biologically stable" carbon control treatment. The chemical characteristics and mineral contents of all amendments were determined each spring prior to land application (Tables 2 and 3).
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We used two types of PMR composts, one with bark as a bulking agent and the other without a bulking agent. The PMR composted with bark as a bulking agent (PMRB) was produced from a recipe of two parts PMR to three parts mixed hardwood bark (by volume). The PMR composted without a bulking agent (PMRC) was obtained from another paper mill in north-central Wisconsin and consisted of 75% primary and 25% secondary wastewater treatment residuals. Both composts were produced with open-air windrow turning methods. Windrows 2 m tall by 3 m wide (and approximately 60–80 m long) were constructed each fall (October–November) and turning took place every 2 to 4 wk with a windrow turner for 5 to 6 mo. The PMRC was probably under anaerobic conditions during composting due to the high moisture content (70% by weight) and fine particle size of the PMR material.
Amendment Application Rates
All PMR amendments were applied annually in April at least 3 wk prior to crop planting. The raw PMR was applied to plots to provide approximately 50 and 100% of the crop's total soil N requirement. We assumed that the total soil N of PMR would range between 1.2 and 1.6% total soil N and that 25% of the PMR total soil N would become available to the crop over the growing season (Peters, 1998). In 1998, PMR was applied at 22.4 (PMR low) and 44.8 dry Mg ha-1 (PMR high) to approximate potato N needs of 168 kg N ha-1 (Kelling and Hero, 1993). In 1999, the PMR high rate was reduced slightly (33.6 dry Mg ha-1) to reflect the reduced N requirements for snap bean. The chemical composition of amendments was used to calculate the actual total mass of C and N added to the top 15 cm of soil (Table 4).
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All amendments were applied moist to plots on a weight basis and spread manually over the entire 35-m2 plot area. A small tractor with a rotovator was used to incorporate the amendments to a soil depth of 15 cm. The nonamended plots were tilled with a rotovator to the same soil depth as the amended plots to eliminate possible confounding effects from tillage.
Planting and Management Practices
In May 1998, plots were planted with five rows of Russet Burbank seed potatoes and harvested mechanically in October 1998. We used the University of Wisconsin Hancock Research Station's recommended practices for weed, pest, and disease control in potato. In June 1999, the field was planted with True Blue snap bean. Weeds were controlled in plots by extensive hoeing and surface rototilling between rows. In early August 1999, the plots were surface rotovated twice to prepare for oat (Avena sativa L.) planting in mid-August.
Soil Measurements
Total soil C and N were measured on 11 occasions between May 1998 and April 2000. A soil probe (3.5-cm diameter) was used to collect approximately 20 soil cores from the surface 15 cm from three blocks of all treatments. In all cases, soil was mixed, plant residue was removed immediately, and the soil was air-dried. After samples were homogenized, a 350-g subsample was collected for subsequent analyses. A portion of this subsample was finely ground with an impact grinder (Shatterbox 8520; Spex CertiPrep, Metunchen, NJ), packed into two ultrapure combustion capsules, and sent to the University of Georgia Institute of Ecology and Analytical Chemistry for total soil C and total soil N dry combustion analysis (NA1500 C/H/N Analyzer; Carlo Erba Instruments, Milan, Italy). We expressed total soil C on an area basis (Mg ha-1), assuming a 15-cm plow depth and using measured bulk density values.
We determined water retention capacity with intact soil cores (7.62 cm high with inner diameter of 7.62 cm) collected in March 1999, June 1999, September 1999, and April 2000 (Klute, 1986). Saturated cores were transferred to a hanging water apparatus as described in McGuire and Lowery (1994). Volumetric water contents of the cores were determined at 0, -2, -5, -8, -12, -20, and -25 J kg-1. Gravimetric soil moisture contents at matric potentials between -100 and -2000 J kg-1 were determined with a thermocouple psychrometer (Decagon Devices, Pullman, WA).
Soil bulk density was measured in all treatments on six dates between April 1998 and April 2000. Bulk density was determined from oven-dried cores (105°C) and then used to convert gravimetric water content to volumetric water content. Soil water release curves were constructed from data collected from the hanging water column and the thermocouple psychrometer. Plant-available water was estimated from release curves and was taken as the difference in volumetric water content between -33 J kg-1 ("field capacity") and -1500 J kg-1 ("wilting point").
Modeling of Irrigation Amounts and Frequency
Amendment effects on irrigation requirements were estimated with a simple soil water balance model and long-term weather dataset. The soil water balance model assumed a single reservoir from which the crop extracted readily available water; that is, soil that held water against significant drainage loss and a tension above which plant growth is inhibited (termed "allowable depletion" by Curwen and Massie, 1984). Increasing the allowable depletion extends the interval between irrigations and improves the likelihood that rainfall can be retained in the soil for crop uptake. We used soil plant-available water values over three dates (pre- and postamendment 1999 and preamendment 2000) to calculate allowable moisture depletion for all treatments.
The long-term weather dataset was obtained from the Midwestern Regional Climate Center for Madison, WI. Parameters included daily rainfall, daily maximum and minimum temperatures, and evapotranspiration. The evapotranspiration values were calculated from daily solar radiation estimates modeled from hourly (or three-hourly) observations of cloudiness and humidity at the Dane County Regional Airport, Madison, WI.
The irrigation scheduling model assumed that a potato crop emerges on 15 May, and that the canopy developed according to a temperature-based model (Connell and Binning, 1999). The soil water balance was calculated daily for the growing season, and irrigation water was applied as needed to prevent depletion beyond the allowable amount. We assumed a 30-cm-deep root zone for potato. We summarized statistics for 27 yr of simulation.
Statistical Analysis
We used the SAS Mixed procedure (repeated measures) (SAS Institute, 1997) to analyze treatment effects on total soil C within a completely randomized complete block design. A chi square test (P < 0.05 with one degree of freedom) was used to evaluate the appropriate covariance structure. We used the SAS Mixed procedure to analyze treatment effects on bulk density and to characterize rate x treatment interactions. We used SigmaStat analysis of variance (SPSS, 1997) to analyze treatment effects on water retention and plant-available water on three dates.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Numerous studies have shown that total soil C increases following introduction of organic amendments such as biosolids (Lindsay and Logan, 1998), biosolids compost (Epstein et al., 1976), and PMR and compost (Aitken et al., 1998). Generally, total soil C decreases over the year following the amendment application and approaches the soil's equilibrium organic matter level. Our results showed that total soil C generally declined in all treatments during the first growing season to near–baseline total soil C levels. However, total soil C in all PMR treatments remained relatively constant after reamendment in the second growing season. These findings also showed that, after only two years of annual amendments to a sandy soil, PMR treatments were able to maintain total soil C levels higher than the nonamended soil. In particular, they suggest that moderate to high amounts of PMRB can build stable organic matter pools that are resistant to decomposition after one year.
We determined the decomposition rate of total soil C in amended soils by regressing the natural log of carbon concentrations in each treatment against the number of days following amendment application. The decomposition period began when amendments were applied and ended with the date prior to the next amendment application (almost 12 mo). The decay rate constants (k) were determined from the slopes of each treatment. The amount of soil carbon (Mg ha-1) remaining after (t) days of decomposition is described by the following equation:
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Our first-amendment-year results indicate that amendments applied at higher rates decomposed faster than amendments applied at lower rates. These findings contradict those of Zibilske (1987), who reported decreasing turnover rates (measured as CO2 evolved) as application rates of primary paper mill sludge increased. Zibilske examined a wide range of application rates; the highest represented approximately twice the amount of material as our high rate. Furthermore, high application rates of primary sludge, normally characterized by low N content (and high C to N ratios), could limit N availability and therefore hinder sludge decomposition over time (Evanylo and Daniels, 1999).
Our study employed lower rates of combined primary and secondary PMR, which typically have greater nitrogen contents than primary sludge alone. The regression analysis between decay constants (k) and the initial C to N ratio of the amendments did not support the concept that C to N ratio influences paper mill sludge decomposition rates in the soil. Other factors affecting carbon quality (e.g., lignin content) or application rate may have affected the rates of PMR, PMRC, and PMRB decay in soil.
Bulk Density
In general, bulk densities of amended plots ranged from 1.0 to 1.5 Mg m-3 and tended to decrease following the annual addition of new organic material (data not shown). Subsequently, they increased over each year, particularly when the soil remained undisturbed. Sampling date, treatment, and interaction of date and treatment significantly affected bulk density over time (P < 0.001). Peat plots showed the greatest decrease in bulk density, approximately 25% lower than control plots (1.0 vs. 1.5 Mg m-3), during autumn 1998 and preamendment 1999 sampling dates. Following the second annual application of amendments (peat not reapplied in 1999), both peat and PMRB plots (both application rates) generally had the lowest bulk densities compared with all other treatments.
Water Retention
One year after amendments were applied, there were significant differences among treatments in the amount of water held at field capacity (33 J kg-1; P < 0.001) (Fig. 2a
; March 1999). With the exception of PMR low, PMRC low, and PMRC high, there was a pattern of greater field capacity (-33 J kg-1) among amended treatments relative to the control; however, only the peat treatments produced statistically significant increases (P < 0.002). There were no significant treatment effects on the amount of water held at wilting point (-1500 J kg-1).
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One year following the second amendment application (April 2000), all treatments increased the amount of water retained at -33 J kg-1, but only the peat, PMRB, and PMRC high treatments held significantly more water than the control (Fig. 2c; P < 0.01). All amendments increased water content at -1500 J kg-1, but only PMRB and high rates of peat and PMRC showed significant increases (P < 0.02).
For years, numerous studies have reported increases in soil water retention following the addition of organic materials (Epstein, 1975). Wei et al. (1985) showed that biosolids additions increased water retention at low tensions in a silty clay loam soil, suggesting an increase in larger pores. Recently, Zibilske et al. (2000) showed that multiple applications of PMR significantly increased soil moisture holding capacity. Municipal solid waste compost addition to sandy soil increased water retention, but did not change plant-available water (Turner et al., 1994).
Our findings suggest that the greatest improvement in water retention resulted shortly after application of PMR, PMRC, and PMRB amendments. Moreover, we observed that the beneficial amendment effects on soil water availability decreased as organic material decomposed over the growing season. Martens and Frankenberger (1992) reported similar findings; increases in moisture content of amended soils diminished as amendments decomposed, but the effects reappeared when amendments were reapplied.
Amended treatments retained more water at -33 J kg-1 and -1500 J kg-1 in the spring of 2000. Given that only peat retained significantly more water at -33 J kg-1 in the first year of amendment application, the second year results suggest that the reapplication of amendments had a cumulative effect on the soil's ability to retain water at -33 J kg-1. The increase in the amount of water held at -1500 J kg-1 between the first and second year provides further support that serial application of amendments had a cumulative effect on soil moisture retention for a wide range of soil-water tensions.
On all three sampling occasions, the amended plots tended to retain more water at -33 J kg-1 than the nonamended control. This supports the positive relationship between organic matter and soil water retention at low tensions (Perfect and Blevins, 1997; Kern 1995). The large increases in water held at -33 J kg-1 in peat and PMRB treatments suggest that we significantly decreased the bulk density and increased the total porosity of the amended soils.
Given that the amount of water retained at -1500 J kg-1 was not affected by the first amendment application, a single application of these materials probably did not affect the number of small pores. However, there was an increase in water content at 1500 J kg-1 after the second-year amendment application. This suggests that organic matter must first decompose to release humic substances that coat soil particles and allow amended soils to retain water as thin films at high matric potentials (Hillel, 1982).
We monitored the change in three soil moisture parameters to understand the mechanism by which the organic amendments improved soil moisture retention. The decrease in bulk density directly increased the soil porosity, thus increasing the saturated water content. Comparisons of the moisture release curves are helpful, but difficult to evaluate quantitatively. We transformed the curves into lines and compared the slopes (Fig. 3
represents data from one regression only). The slope of the line relating ln 
and ln(
/s) was obtained by taking a natural log transformation of the following equation:
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is matric potential, e is air entry potential, is water content, s is saturated water content, and -b is the exponent of the moisture release equation (Campbell, 1974).
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e, is related to the mean pore size and is estimated by the y intercept. One year following the second amendment application, the slope (b) values were greater than those generated from the first-year water retention curves (data not shown). The slope increase suggests that we are increasing the soil pore size distribution by adding organic matter or supporting the development of small pores following the application of organic material. Furthermore, the estimated air entry potential tended to become more negative one year following the second amendment application. This suggests that the presence of small pores following the addition of organic material decreased the mean soil pore size. The increase in magnitude of the slope and the observed shift in the air entry potential from the first year to the second year suggest that additions of organic matter to coarse-textured soils increase the amount of water retained at the wilting point. From a soil textural standpoint, the shift in air-entry potential, the increase in slope (b), and the increase in saturated water content suggest that this loamy sand soil is behaving more like a sandy loam following two years of amendment applications.
Plant-Available Water
Plant-available water was significantly different among treatments (P < 0.001) one year after the first amendment application (preamendment 1999; Fig. 4)
. The high application rates slightly increased plant-available water relative to the control, but significant increases were only seen in the peat-amended soils (P < 0.001). Soils amended with PMR high and Peat high had significantly more PAW than those amended with PMR low and Peat low (P < 0.025; only high-rate PAW data is shown in Fig. 4). Shortly following the second application, the percentage of soil PAW was 15 to 45% greater in amended soils compared with the nonamended control; all increases were statistically significant (P < 0.03) except for PMRC low. In April 2000 (preamendment 2000), one year following the second amendment application, the PAW in amended treatments was 5–45% greater than the control. With the exception of PMR and the PMRC low (P = 0.35), these increases were statistically significant.
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The increase in water held at -33 J kg-1 is related to increased total porosity, which simply allowed the soil to hold more water. However, the increase in small pores improved the soil's ability to retain water at -1500 J kg-1. Evidence of greater pore-size distribution and a shift toward smaller pores suggests that greater moisture retention at -33 J kg-1 might be offset by greater moisture retention at -1500 J kg-1.
The decline in PAW with PMR and PMRC between the first and second amendment applications supports the need for annual additions of these materials to maintain elevated PAW relative to nonamended soils. The cumulative effects of repeated additions of PMRB on PAW suggest that certain composts might sustain elevated PAW beyond one year. Soils amended with PMRB had lower bulk densities than PMR and PMRC soils, leading to greater soil porosity. The increase in the amount of water retained at -33 J kg-1 was larger than the increase in the amount of water held at -1500 J kg-1, thus maintaining improved PAW.
Relationship between Plant-Available Water and Total Carbon
There was a positive linear relationship between total soil C and PAW on the three dates examined and the relationship strengthened over the two years of study (Fig. 5)
. One year following the first application of amendments, there was a positive linear relationship between total soil C and PAW, but the correlation was low (Fig. 5a; R2 = 0.18, P < 0.001). Immediately following the second application (May 1999), all amended treatments had greater PAW and total soil C relative to the control (Fig. 5b; R2 = 0.44, P < 0.001). The linearity of the total soil C–PAW relationship improved after two years of annual amendment additions (Fig. 5c; R2 = 0.83, P < 0.0001). We attribute the low correlation in Year 1 to the very small range of total soil C, the independent variable in the regression. Following the addition of more organic matter in Year 2, the range of total soil C increased by approximately a factor of six, thus improving the correlation.
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Relationship between Total Carbon and Irrigation Scheduling
All amended treatments reduced the average amount of irrigation water required over the potato growing season by 4 to 30% compared with the nonamended control (Table 6). The reduction in the amount of irrigation water was significant in plots amended with peat and PMRB (P < 0.01), along with the high rates of PMR and PMRC (P < 0.06). There were no significant rate effects on the amount of irrigation water needed (P < 0.23).
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The positive effects of PMR treatments on total soil C and PAW did not manifest themselves until the second year of amendment application; accordingly, their effects on crop growth did not appear until the second crop in the rotation. In the first year of study, there were no statistical differences in potato yields and quality (no. Grade A potatoes) among all the PMR treatments and the nonamended control (Foley, 2001). However, in the second year, all PMR amendments at both application rates increased total snap bean yields relative to the nonamended control.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The amendments increased total soil porosity, and thus increased the amount of water retained at field capacity. The breadth of the soil pore size distribution increased and the mean pore size decreased following the addition of organic material. This increased the amount of water retained at the wilting point. Since the increase in the amount of water retained at field capacity was larger than the increase in water retained at the wilting point, certain PMR treatments increased PAW. From a soil textural standpoint, the decrease in air-entry potential, increase in slope (b), and an increase in saturated water content suggest that our loamy sand study soil is behaving more like a sandy loam following two years of amendment applications.
The amount of retained soil water and PAW in amended plots tended to decrease as amendment material decomposed over time. The relationship between total soil C and PAW was positive and linear over all dates. However, the beneficial effects of PMR and PMR composts on soil water retention decreased as organic material decomposed. Annual additions may be required to maintain favorable effects on soil physical properties. Plots amended with PMRB maintained positive effects on soil physical properties beyond one year, suggesting it could be applied less frequently without sacrificing its beneficial effects. The PMR and PMR compost improvements in PAW translated to reductions in irrigation amounts and frequencies, suggesting that PMR amendments increased water use efficiency.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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= 0.05). Error bars represent ± one standard deviation. PMR, paper mill residuals; PMRB, paper mill residuals composted with bark; PMRC, paper mill residuals composted without a bulking agent.
