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a Dep. of Crop, Soil, and Environmental Sciences, Univ. of Arkansas, Fayetteville, AR 72701
b Graduate Research Assistant, Dep. of Crop, Soil, and Environmental Sciences, Altheimer Lab, 1366 W Altheimer Dr., Fayetteville, AR 72704
c Associate Professor, Dep. of Crop, Soil, and Environmental Sciences, Altheimer Lab, 1366 W Altheimer Dr., Fayetteville, AR 72704
* Corresponding author (kbrye{at}uark.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: EC, electrical conductivity • LAI, leaf area index • OC, organic C • PTBS, Pine Tree Branch Station • RREC, Rice Research and Extension Center • VWC, volumetric water content
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Slaton et al. (2004) concluded that transport of litter-derived nutrients out of western Arkansas is critical to establishing a balance between inputs and removals of nutrients. It was also suggested that the eastern one third of Arkansas, which is a predominantly row-crop agricultural region, could benefit greatly from litter-derived N and P due to the overall nutrient deficit from high nutrient removals by row crops (Slaton et al., 2004). Addition of organic materials may improve the fertility and physical properties of low-organic-matter soils. Poultry litter additions to agricultural soils in eastern Arkansas have been shown to decrease soil surface bulk density in silt-loam but not silty-clay soils (Brye et al., 2004). The inherent barriers inhibiting relocation of large quantities of litter to eastern Arkansas have been litter's relatively low fertilizer value coupled with prohibitively high transportation costs. Some have suggested that pelletized litter would be easier to package and transport than fresh or composted litter, a process that is being done with poultry litter from Delaware, Maryland, and Virginia (Hamilton and Sims, 1995).
In 2004, Arkansas ranked first among producer states, with 45% of the total US rice production (Agricultural Statistics Board, 2005). In Arkansas, the annual planted-rice area averages 600 000 ha (Arkansas Statistical Office, 2005), which may benefit from litter-derived nutrients and organic matter. Miller et al. (1990, 1991) reported that rice and soybean (Glycine max L.) yields in eastern Arkansas increased with the use of poultry litter compared with inorganic fertilizers on recently land-leveled or graded soils.
Few studies have investigated the effects of poultry litter on nongraded soils used for rice production in the mid-southern USA. Decomposition of poultry litter and soil C and N dynamics need to be evaluated to ascertain the potential benefits of poultry litter use in rice culture as a fertilizer N, P, and/or K source or as an organic soil amendment to improve soil physical properties. Therefore, the objectives of this study were to (i) evaluate the effect of poultry litter form (fresh and pelletized) and five application rates (34–269 kg N ha–1) on soil surface CO2 flux, total soil N and organic C, and inorganic soil N before flood establishment and (ii) evaluate the effect of soil temperature and moisture on soil surface CO2 flux in two silt-loam soils to determine mechanisms and kinetics of the mineralization and release of N from fresh and pelletized litter in rice soils. Due to larger surface area and a more labile organic substrate, it was hypothesized that fresh poultry litter would decompose faster, thus result in a higher soil surface CO2 flux and greater inorganic N accumulation, than pelletized poultry litter and that soil surface CO2 flux would increase as litter rate increased. It was also hypothesized that near-surface soil temperature and moisture are significant factors controlling soil surface CO2 flux in the pre-flood period of the rice growing season.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Experimental Design and Treatments
Experimental field plots were established at each study location based on a randomized, complete-block design with four replications. Treatments were arranged in a 2 x 5 factorial structure consisting of two litter forms, fresh and pelletized (Harris Litter, Bentonville, AR), applied at five rates equivalent to 34, 67, 134, 202, and 269 kg total N ha–1 contained in each litter form along with an unamended control that received no N. Fresh litter was obtained from a commercial broiler production unit located at the University of Arkansas Experiment Station, Savoy, AR and consisted of uncomposted, clean-out material, including rice hulls and sawdust as bedding, after 12 mo of broiler production. The final application rates on a fresh-weight basis were 903, 1807, 3613, 5420, and 7226 kg ha–1 of fresh and 840, 1680, 3360, 5040, and 6720 kg ha–1 of pelletized poultry litter. Treatments were applied to 2- by 4.9-m plots on 19 and 20 Apr. 2004 at RREC and PTBS, respectively. The study area at each location was <0.1 ha.
After manual surface application of litter treatments, P and K fertilizer was broadcast on all plots at each location at rates of 20 kg P ha–1 and 50 kg K ha–1, and a rototiller was used to incorporate litter to a depth of 10 cm within a few hours. The soil of the unamended control plots also received P and K but no N and was incorporated by rototilling. ‘Wells’ rice was drill seeded at 112 kg seed ha–1 immediately after litter incorporation at both locations, resulting in nine rice rows per plot. The flood was established on 3 and 8 June at PTBS and RREC, respectively.
Poultry Litter Characterization
Three subsamples of each litter form were chopped for chemical characterization according to recommended methods for manure analysis (Peters, 2003). All chemical analyses were performed on moist litter, and results were expressed on a dry-weight basis. Litter pH and electrical conductivity (EC) were determined potentiometrically on a 1:2 litter/water mixture. Litter subsamples were digested in a mixture of nitric and hydrochloric acid at low temperature and treated with hydrogen peroxide, and multi-element (total recoverable P, K, calcium [Ca], magnesium [Mg], sodium [Na], sulfur [S]) analysis was performed using an inductively coupled argon-plasma spectrophotometer (ICPS; CIROS CCD model; Spectro Analytical Instruments, Marlborough, MA). Total litter C and N were determined by high-temperature combustion with a LECO CN-2000 analyzer (LECO Corp., St. Joseph, MI). Litter subsamples were also extracted with potassium chloride (KCl) for colorimetric determination of inorganic NO3–N and NH4–N. Table 1 summarizes the chemical composition of fresh and pelletized litter at the time of application.
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Soil surface CO2 flux was measured once before litter application and incorporation (15 April) and four times approximately weekly in May 2004 between litter incorporation and flood establishment (5, 13, 19, and 31 May). Soil surface CO2 flux was measured with a LI-6400 portable CO2 infrared gas analyzer (Li-Cor Inc., Lincoln, NE) equipped with a 10-cm-diameter soil respiration chamber (LI-6400-09; Li-Cor, Inc.) (Wagai et al., 1998; Brye et al., 2002).
Before the first measurement date, one thin-walled (3.2 mm) polyvinyl chloride ring (i.e., collar; 10-cm inside diameter x 5 cm tall) was inserted approximately 2 cm into the soil at a random location in each plot. The collars were constructed such that the soil respiration chamber would fit snugly on top of each collar when the measurements were being performed. After completing each set of individual measurements at a location, the collars were inserted into a different random location in each plot and allowed to equilibrate until the next measurement date. Each set of measurements were conducted between 0700 and 1000 h at PTBS and between 1100 and 1400 h at RREC.
Soil temperature and volumetric water content (VWC) were measured, and soil samples were collected along with each soil surface CO2 flux measurement. Soil temperature was measured within 5 cm of the outside perimeter of each collar at depths of 2.5 and 10 cm using a long-stem soil thermometer. Volumetric soil water content was measured in the 0- to 6-cm depth inside the collar immediately after the CO2 flux measurement was completed using a Theta Probe (model TH2O; Dynamax, Houston, TX), which records dielectric voltage readings and converts them to volumetric water contents using a soil-specific calibration equation.
After each soil surface CO2 flux measurement, three soil cores were collected from the 0- to 10-cm depth from within each CO2 flux collar and combined into one composite sample per plot. Soil was dried, crushed, and sieved to pass a 2-mm mesh screen. Subsamples were analyzed for inorganic soil NO3–N and NH4–N by KCl extraction and total C and N as previously described.
Plant Measurements
Leaf area index (LAI) was measured nondestructively on rice seedlings using a LI-COR LAI-2000 plant canopy analyzer (LI-COR, Inc., Lincoln, NE) (Wells and Norman, 1991) the day of flood establishment (3 June) at PTBS and 4 d before flooding (4 June) at RREC. One measurement was conducted in the early evening with uniformly overcast sky conditions approximately 1 m into the middle row of each nine-row plot.
A 0.91-m length of the first inside row of each plot was cut at the soil surface and dried to a constant weight for pre-flood aboveground dry matter determination. Dried plant material was ground and sieved to pass a 2-mm mesh screen, and total N concentration was determined by high-temperature combustion with a LECO CN-2000 analyzer. Aboveground dry matter and total N concentration were used to calculate aboveground N uptake.
Statistical Analyses
To ascertain differences between litter forms and between locations, t tests were performed on litter and initial soil chemical properties (Minitab, 2000).
A three-way analysis of variance (ANOVA) was conducted to determine the effect of location, litter form, and rate on soil surface CO2 flux, 2.5- and 10-cm soil temperature, VWC, total soil N and OC, and inorganic soil N (SAS, 2002). To simplify the model, initial analyses were conducted separately by sample date. A similar ANOVA was conducted to determine the effect of location, litter form, and rate on rice LAI, dry matter, tissue N concentration, and aboveground N uptake. Treatment means were separated by least significant difference at LSD0.05. Due to dissimilar sample sizes between litter rates (n = 16 for each litter rate) and the unamended control (n = 8) when pooled across location and litter form, two different LSD0.05 values were calculated to compare a single litter rate to the unamended control and to compare among littered treatments. Linear regression analyses were performed to ascertain trends over time for soil surface CO2 flux, total soil N and OC, and inorganic soil N (Minitab, 2000).
Linear correlations were performed with data pooled across location, sample date, and rate to ascertain the potential effect of poultry litter form on the linear relationship between soil surface CO2 flux and 2.5- and 10-cm soil temperature and VWC (Minitab, 2000). Based on significant correlations and correlation differences between litter forms, an analysis of covariance was performed with data pooled across location, sample date, and rate to directly evaluate the effect of litter form on the linear relationship between soil surface CO2 flux and 2.5- and 10-cm soil temperature and VWC (SAS, 2002). Nonlinear and multiple regression analyses were also performed with data pooled across location, sample date, litter form, and rate to evaluate the linear and quadratic effects of 2.5- and 10-cm soil temperature and VWC separately and combined to explain soil surface CO2 flux variation (Minitab, 2000).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Averaged across location and litter form, soil surface CO2 flux was greater from the highest litter rate (269 kg total N ha–1) than from the unamended control. Soil surface CO2 flux from all other litter rates did not differ from the unamended control. Similarly, comparing only among littered treatments, soil surface CO2 flux was similar between the two highest litter rates (202 and 269 kg total N ha–1) but was greater than the three lowest litter rates (34, 67, and 134 kg total N ha–1), among which soil surface CO2 flux did not differ.
Soil surface CO2 flux generally differed between locations throughout the 1-mo before flood establishment (Table 3). Soil surface CO2 flux was higher at RREC than at PTBS on three of the four post-litter sample dates (on 19 May soil surface CO2 flux was similar at both locations) (Fig. 1). Despite lower initial total soil OC and NO3–N concentrations at RREC than at PTBS (Table 2), the generally higher soil surface CO2 fluxes at RREC were likely the result of consistently higher 2.5- and 10-cm soil temperatures across all four post-litter sample dates at RREC than at PTBS (Fig. 2), indicating that soil surface CO2 flux is likely, at least partially, controlled by soil temperature.
There was a significant rate effect on 10-cm soil temperature on 5 May (P = 0.024) and on 2.5-cm soil temperature on 31 May (P = 0.010), but not on VWC (Table 3) likely due to increased surface moisture variations from differential drying. However, because there was less than a 0.5°C difference between highest and lowest 2.5- or 10-cm soil temperatures on either sample date, this slight temperature difference was likely biologically insignificant and was likely not the factor responsible for the significant rate effect on soil surface CO2 flux. Despite significant location and litter rate effects, there were no significant trends in soil surface CO2 flux over time throughout the 1-mo post-litter measurement period for any litter rate at either location (Fig. 1).
When averaged across locations to capture the widest possible variability, simple linear correlations indicated that soil surface CO2 flux from pelletized and fresh litter were positively correlated with 2.5- and 10-cm soil temperature (0.21 < r < 0.24; P < 0.003), a result similar to other field studies (Rochette et al., 1991; Alvarez et al., 1995; Luo et al., 2001; Tufekcioglu et al., 2001). Soil surface CO2 flux from only the fresh litter was negatively correlated with VWC (r = –0.16; P = 0.027). These results are similar to past studies reporting that soil temperature has a greater effect on soil surface CO2 flux than soil moisture (Hendrix et al., 1988; Fortin et al., 1996; Wagai et al., 1998). The negative correlation with VWC, indicting that soil surface CO2 flux decreased as soil moisture increased, is somewhat counterintuitive because other studies have indicated a positive relationship between soil respiration and soil moisture (Wildung et al., 1975; Kessavalou et al. (1998a,b; Wagai et al., 1998). However, few past studies have been conducted under similar temperature and moisture conditions as exist between April and June in eastern Arkansas. When included with their linear term to explain soil surface CO2 flux variations, the quadratic terms for 2.5- and 10-cm soil temperature and 0- to 6-cm volumetric soil water content were not significant.
Correlation results indicated that litter form may have a differential effect on the relationship between soil surface CO2 flux and soil temperature and moisture. However, neither the slopes nor intercepts for the separate linear relationships between soil surface CO2 flux and 2.5- and 10-cm soil temperature and VWC differed by litter form. Data were combined across litter form to yield significant, though poor, relationships between soil surface CO2 flux and 2.5-cm (r2 = 0.048; P < 0.001) and 10-cm soil temperature (r2 = 0.049; P < 0.001) only. When data were combined across litter form, soil surface CO2 flux and VWC were not related. When their linear terms were included together in a multiple regression model, neither 2.5- or 10-cm soil temperature nor VWC were significant and together explained only 5.2% of the variation in soil surface CO2 flux. de Jong et al. (1974) also reported that the addition of soil moisture did not improve the multiple regression model explaining soil respiration variability for cereal or fallow cover crops in Saskatchewan, Canada. The lack of significant temperature and moisture effects indicate that other factors are controlling soil surface CO2 flux early in the rice growing season on silt-loam soils in eastern Arkansas.
Soil Organic Carbon and Total Soil Nitrogen
Due to inherent fertility differences, soil OC concentrations were consistently greater (Table 3) at PTBS than RREC on each post-litter sample date (Fig. 3
). Total soil N concentration was greater at PTBS than RREC only on the first sample date after litter application and was similar between locations on the other sample dates (Table 3). Because there were no location by litter form or rate interactions on any sample date (Table 3), it is likely that location differences were due to inherent soil fertility differences on any given sample date and not due to a consistent differential location response to litter application.
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Soil OC and total soil N differed by litter rate on the first two sample dates (5 May and 13 May) after litter application and incorporation (Fig. 3). On 5 May, soil OC and total soil N were similar between litter applied at 34 kg total N ha–1 and the unamended control, but soil OC and total soil N were greater in the litter rates than that in the unamended control. Soil OC and total soil N were also greater in the highest litter rate than in the two lowest litter rates, which did not differ. On 13 May, soil OC in the highest litter rate was greater than in the unamended control, but soil OC was similar between the unamended control and all other litter rates. Total soil N did not differ between the unamended control and littered treatments. Soil OC was similar among the three lowest litter rates but was lower than that from the highest litter rate. Total soil N was similar between the two highest litter rates and higher than that from the lowest litter rate.
Soil OC differed by rate 29 d after litter application (19 May) (Fig. 3). Soil OC in the two highest litter rates was greater than in the unamended control but was similar between the unamended control and the three lowest litter rates. Similar to the previous sample date, soil OC did not differ between the two highest litter rates and was greater than in the lowest litter rate. Soil OC and total soil N concentrations were unaffected by litter rate on 31 May (41 d after litter application) (Table 3).
Despite several significant litter rate effects across the four post-litter samples dates, there were generally few significant trends over time for soil OC or total soil N concentrations by location and litter rate separately when averaged across litter form. At PTBS, soil OC in the 134 kg total N ha–1 litter rate decreased throughout the 6-wk measurement period (Fig. 3). At RREC, soil OC in the unamended control and total soil N in the 202 and 269 kg total N ha–1 litter rates decreased throughout the 6-wk measurement period (Fig. 3). However, the slopes and intercepts for the 202 and 269 kg total N ha–1 litter rates characterizing the significant trends in total soil N over time did not differ. The larger number of significant time trends at RREC may be related to the lower overall inherent soil fertility level (Table 2), resulting in RREC being slightly more responsive to litter application than PTBS.
When combined across location, sample date, and litter form and rate, soil surface CO2 flux from the fresh litter was negatively correlated with soil OC (r = –0.23; P = 0.003) but not total soil N. Soil surface CO2 flux from the pelletized litter was unrelated to soil OC or N, indicating that litter form may affect these relationships. However, neither the slopes nor intercepts for the separate linear relationships between soil surface CO2 flux and soil OC or total soil N differed by litter form.
Inorganic Soil Nitrogen
Similar to pre-litter soil conditions, averaged across litter form and rate, soil NO3–N was greater and soil NH4–N was lower at PTBS than RREC on 5 and 13 May (Table 3). The generally wetter and cooler soil environment at PTBS compared with RREC during this time (Fig. 2) likely contributed to differences in inorganic soil N among locations. In contrast to pre-litter soil conditions, soil NO3–N was greater and soil NH4–N was lower at RREC than PTBS on 19 May (Table 3). On 31 May, soil NO3–N concentrations were similar between locations, but soil NH4–N was greater at RREC than PTBS (Table 3). However, inorganic soil N fluctuations did not coincide well with soil moisture and temperature fluctuations during this time.
Soil NO3–N and NH4–N differed by litter treatment, and both had significant location by litter form interactions on the first post-litter sample date (5 May) (Table 3). Soil NO3–N was similar between the two litter forms but greater than the unamended control at RREC, whereas soil NO3–N was greater in the soil receiving pelletized than in the soil with fresh litter, and both were greater than in the unamended control soil at PTBS (Fig. 4 ). Soil NH4–N was similar among the two litter forms but greater than the unamended control soil at PTBS, whereas soil NH4–N was higher in the soil with pelletized than with fresh litter, and both were greater than the unamended control soil at RREC on 5 May (Fig. 4).
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Soil NO3–N differed by litter treatment on 19 and 31 May (Table 3). On 19 May, soil NO3–N was greater in each littered treatment than in the unamended control and was greater in the pelletized than fresh litter at RREC, whereas soil NO3–N was greater in only the pelletized litter than in the unamended control and was similar between litter forms at PTBS (Fig. 4). On 31 May, soil NO3–N was greater in each litter form than in the unamended control at both locations, whereas soil NO3–N was also greater in the pelletized than fresh litter at RREC and similar between litter forms at PTBS (Fig. 4).
Soil NO3–N decreased throughout the measurement period in the unamended control at PTBS but not at RREC (Fig. 4). At RREC, soil NO3–N increased during the 1-mo measurement period in the pelletized litter treatment but not at PTBS (Fig. 4). The pelletized litter used in this study also had nearly a threefold greater NO3–N concentration than did the fresh litter (Table 1). Despite soil surface CO2 flux remaining relatively steady throughout the 1-mo measurement period, NO3–N tended to accumulate in the soil as a likely mineralized by-product from the decomposition of the pelletized litter, indicating that decomposition rates may differ between pelletized and uncomposted fresh litter under field conditions. It is possible that the disintegration of litter pellets and accessibility of the substrate may have caused the delayed mineralization of litter N. A corresponding decline over time in soil NH4–N was also observed at RREC (Fig. 4). Meanwhile, both NO3–N and NH4–N showed an overall decline over time, decreasing to soil concentrations equal to those found in the unamended control plots. Gordillo and Cabrera (1997) reported that the mineralization of organic N in poultry litter decreased as soil pH increased, which may partially explain the apparent litter decomposition response difference between locations in this study, where soil pH in the top 10 cm was 7.3 at PTBS and 6.4 at RREC.
In addition to significant location and litter form effects and when averaged across location and litter form, soil NO3–N and NH4–N also differed among litter rates on each post-litter sample date with no litter form by rate interaction on any sample date (Table 3 and Fig. 5 ). On 5 May, 15 d after litter application, averaged across location, soil NO3–N and NH4–N tended to increase as litter rate increased, differing among litter rates as follows: 34 = 67 < 134 = 202 < 269 kg total N ha–1. However, there was also a significant location by rate interaction on 5 May for soil NO3–N and NH4–N, where soil NO3–N was greater in the 67, 134, 202, and 269 kg total N ha–1 litter rates at PTBS than in the same rates at RREC, whereas soil NH4–N was greater in the 134, 202, and 269 kg total N ha–1 litter rates at RREC than in the same rates at PTBS.
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On 19 May, 29 d after litter application, soil NO3–N tended to increase as litter rate increased, but the increase differed by location, where soil NO3–N was greater in the 134, 202, and 269 kg total N ha–1 litter rates at RREC than in the same rates at PTBS. Soil NH4–N was greater in all litter rates than in the unamended control and also tended to increase as litter rate increased with soil NH4–N, being similar among the 34, 67, and 134 kg total N ha–1 litter rates but lower than that from the 202 and 269 kg total N ha–1 litter rates, which did not differ. On 31 May, 41 d after litter application, soil NO3–N was greater in the 134, 202, and 269 kg total N ha–1 litter rates, and soil NH4–N was greater in the 202 and 269 kg total N ha–1 litter rates than in the unamended control, and soil NO3–N and NH4–N tended to increase as litter rate increased.
The general increase in soil inorganic N over and above the levels found in the unamended control, maintained throughout the 1-mo study period, seemed to be in proportion to the quantity of unpelletized fresh or pelletized litter added (Table 1 and Fig. 5). However, only at PTBS was there a detectable decline in soil NO3–N (Fig. 5).
Rice Responses to Poultry Litter
By 45 d after litter application (3 and 4 June), rice LAI was greater in all litter rates, except for the 34 kg total N ha–1 rate, than in the unamended control and tended to increase as litter rate increased (Fig. 6
). However, the increase in LAI differed by location (Table 4), where LAI was greater in the 134, 202, and 269 kg total N ha–1 litter rates at RREC than in the same rates at PTBS (Fig. 6).
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With aboveground N uptake averaging between 10 and 30 kg ha–1 (Fig. 6 and 7) for the littered treatments and soil NO3–N and NH4–N in the top 10 cm averaging about 10 and 6 kg ha–1, respectively (Fig. 4 and 5), before flooding, there is a discrepancy of between 8 and 223 kg N ha–1 between the litter-applied N and the N that can be accounted for in the top 10 cm of soil and in the rice plants at the 4- to 5-leaf stage. This disparity indicates that a significant proportion of the litter-applied N was lost in the 1-mo period after litter application due to leaching below 10 cm or denitrification shortly after rainfall events. Volatilization losses were assumed negligible because the litter was incorporated.
Although it is not obvious from litter decomposition dynamics or decomposition products, such as inorganic soil N, rice response data indicate that litter rates >135 kg total N ha–1 (3500 kg ha–1) created a positive growth response before flooding. Dry matter and N uptake data also indicated that higher rates of pelletized litter resulted in a greater positive rice growth response than similar rates of fresh litter. However, poultry litter, fresh or pelletized, applied at relatively high rates (>134 kg total N ha–1), likely has some (small) fertilizer-N value for rice production in eastern Arkansas, at least as a starter-N source. The P- and K-fertilizer value of poultry litter may be greater than that for N due to the smaller plant requirements for these elements. Even if poultry litter were used only as an organic soil amendment and not as a primary nutrient source, long-term benefits, such as improved soil tilth, increased crop yields (Miller et al., 1990, 1991), SOM (Sommerfeldt et al., 1988; Kingery et al., 1993), infiltration, water-holding capacity, and decreased soil bulk density (Brye et al., 2004), would likely result.
Implications of Rice Culture
Common rice cultural practices may be contributing to reduced effects of soil temperature and moisture, which have often been shown to significantly control soil respiration. After soybean harvest, it is commonplace for many Arkansas producers to leave a field fallow and unmanipulated over the winter months. Subsequently, before rice planting the following year, a field may be tilled multiple times to fill in ruts that may have been left from the previous year's harvest and to prepare as smooth of a seedbed as possible. The multiple tillage passes through a field nearly eliminates any soil structure in the plow layer; hence, microorganism habitat is significantly reduced. Furthermore, continuous annual tillage practices of this nature have resulted in many eastern Arkansas agricultural soils with low SOM (i.e., <1%). Therefore, without a sufficient baseline population of soil microorganisms to multiply while processing newly added and readily decomposable organic material, such as poultry litter, soil biological factors (i.e., microbial biomass), induced by field management practices, may exert greater control on soil surface CO2 flux than environmental factors (e.g., soil temperature and moisture). Future field studies evaluating decomposition dynamics of poultry litter and its fertilizer value for crop production in eastern Arkansas need to include evaluations of soil microbial biomass as the mechanism responsible for organic matter turnover and nutrient release.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication June 24, 2005.
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