Soil Science Society of America Journal 67:146-155 (2003)
© 2003 Soil Science Society of America
DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY
Soil Aggregation and Carbon and Nitrogen Pools under Rhizoma Peanut and Perennial Weeds
U. M. Sainju*,
T. H. Terrill,
S. Gelaye and
B. P. Singh
Agricultural Res. Stn., Fort Valley State Univ., 1005 State University Drive, Fort Valley, GA 31030
* Corresponding author (sainjuu{at}fvsu.edu)
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ABSTRACT
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Roots of rhizoma peanut [(Arachis glabrata Benth.), a warm-season perennial legume forage] and perennial weeds may influence soil aggregation and associated C and N pools. We compared aggregate-size distribution and concentrations of organic C and N, NH4–N, NO3–N, potential C and N mineralization (PCM and PNM), microbial biomass C and N (MBC and MBN), and particulate organic C and N (POC and PON) in whole-soil and aggregates under 10-yr-old rhizoma peanut and perennial weeds [dominated by henbit (Lamium amplexicaule L.) and cut-leaf evening primrose (Oenothera laciniata Hill)]. Field plots were established on a Norfolk loamy fine sand in April 1991 in central Georgia. While soil aggregation and associated C and N pools were not influenced by treatments, whole-soil NH4–N and PON at the 0- to 15-cm, PCM at the 30- to 90-cm, and MBC at the 0- to 30-cm depth were 28 to 100% greater under peanut than under weeds. Under both treatments, the amount of soil present in the 4.75- to 2.00-mm aggregate-size class and mean-weight diameter of aggregates were 21 to 47% greater at 15- to 60- than at 0- to 15-cm, indicating improved aggregation in the subsurface compared with the surface soil. At 0 to 15 cm, concentrations of organic C and N, MBC, MBN, POC, and PON were greater in the <0.25-mm than in the 4.75- to 0.50-mm size class. At 0 to 30 cm, NO3–N was greater in the 4.75- to 0.85-mm than in the 0.50- to 0.25-mm size class. Although treatments did not influence soil organic C and N levels, rhizoma peanut may increase microbial activities and labile N pools compared with perennial weeds. Rhizoma peanut and perennial weeds may improve aggregation in the subsurface compared with the surface soil and C and N pools in aggregates <0.25 mm in diameter.
Abbreviations: MBC, microbial biomass C • MBN, microbial biomass N • PCM, potential C mineralization • PNM, potential N mineralization • POC, particulate organic C • and PON, particulate organic N
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INTRODUCTION
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GRASSLAND SOIL has been identified as one of the potential sites to sequester atmospheric CO2 in the terrestrial ecosystem for helping to reduce the deleterious effects of greenhouse gas (Bouwman, 1990). This is because grassland soil has higher organic C concentration than in agricultural soil due to continuous C input from above and belowground plant biomass and reduced rate of mineralization from decreased soil disturbance (Elliott, 1986; Cambardella and Elliott, 1992; Six et al., 1998). Plant C is sequestered in the soil through decomposition and conversion into soil organic C in aggregates. As a result, addition of C and N from plant residues may improve soil quality and productivity by improving microbial activities, N mineralization, and aggregation and increasing C and N storage in soil. Perennial legume forages, such as rhizoma peanut that survive well in the Coastal Plain of southeast USA (Prine et al., 1986; Terrill et al., 1996, 2000), may have an additional advantage of fixing atmospheric N, thereby also enriching soil N and improving fertility.
While much has been known about aggregation in the surface soil, relatively little information exists about aggregation, microbial activities, and C and N pools in the subsurface soil. Aggregation improves water infiltration capacity and root growth in the surface and subsurface soils and reduces erosion primarily in the surface soil in agricultural areas where tillage and other cultural practices disturb the land. Perennial forages can be grown in areas that are not suitable for growing agricultural crops, such as denuded or sloping lands where erosion from both surface and subsurface soils can occur. In such areas, improved aggregation is needed not only in the surface but also in the subsurface soil to increase water infiltration capacity and root growth and reduce soil erosion. Even in flat lands where forages are grown, subsurface aggregation may be needed to improve water percolation and root growth of deep rooted species. Soils under perennial forages are usually undisturbed and roots continue to grow horizontally and vertically into the soil. Roots of rhizoma peanut have been found to be well distributed to a depth >60 cm (Terrill et al., 2000). Roots improve aggregation by enmeshing soil particles together and by increasing microbial biomass which produce polymers that act as binding agents (Tisdall and Oades, 1979; Jastrow et al., 1998). Furthermore, as roots of perennial forages grow and die each year, C and N from the residue are continuously added to the soil. Rhizodeposition, such as root exudates, may be a significant source of soil organic C (Buyanovsky et al., 1986; Balesdent and Balabane, 1996). As a result, roots may play a dominant role in soil C and N cycles (Wedin and Tilman, 1990; Gale et al., 2000; Puget and Drinkwater, 2001) and may have relatively greater influence on soil organic C and N than the aboveground plant biomass (Milchumas et al., 1985; Boone, 1994; Norby and Cotrufo, 1998). Because legumes fix N from the atmosphere, their roots may supply additional N to the soil, thereby enriching soil N, increasing microbial activities, and improving soil fertility. To measure these potentials, soil N and C pools under legumes were compared with those under nonlegumes or some native species, such as naturally occurring weeds within the region. In the long-term (>10 yr) productivity of rhizoma peanut where aboveground biomass is harvested to feed the animals, roots may improve soil quality and productivity compared with perennial weeds by improving soil aggregation, increasing microbial activities, and enriching soil C and N.
The distribution of microbial biomass and activities among soil aggregates is heterogeneous (Gupta and Germida, 1988; Seech and Beauchamp, 1988; Miller and Dick, 1995). While some researchers have observed greater microbial biomass and activities in macroaggregates (>0.25 mm) than in microaggregates (Gupta and Germida, 1988; Franzluebbers and Arshad, 1997), others have found greater activities in microaggregates (<0.25 mm) than in macroaggregates (Seech and Beauchamp, 1988; Jastrow et al., 1998). Still others have reported greater microbial biomass and activities in the intermediate size (0.25–1.00 mm) aggregates than in the >1.00-mm or <0.25-mm size class (Mendes et al., 1999; Schutter and Dick, 2002). Little is known about the distribution of microbial biomass and activities and C and N pools among aggregates of undisturbed surface and subsurface soils under forages. An examination of microbial biomass and activities and C and N pools within aggregates as influenced by forage species will provide insight to better understand the growth of microbial communities, turnover of organic matter, and N availability in soil.
We measured aggregate size distribution and NH4–N, NO3–N, organic C and N, PCM, PNM, MBC, MBN, POC, and PON concentrations in whole-soil and aggregates from the 0- to 90-cm depth under a 10-yr growth of rhizoma peanut and adjacent perennial weeds. While total organic C and N are regarded as recalcitrant pools of soil C and N that change slowly with time, PCM, PNM, MBC, and MBN are considered labile pools that change seasonally (Franzluebbers and Arshad, 1997; Franzluebbers et al., 1999), and influence aggregation (Jastrow et al., 1998; Martens, 2000). Similarly, POC and PON are considered as intermediate pools for changes of soil C and N with time that provide substrates for microbes (Beare et al., 1994; Franzluebbers et al., 1999; Six et al., 1999) and influence aggregation (Gale et al., 2000). Available forms of N that influence plant growth are NH4 and NO3–N. Because of deep root growth and N fixing ability, we hypothesized that long-term productivity of rhizoma peanut might improve aggregation and associated C and N pools in both surface and subsurface soils compared with perennial weeds. Our objectives were to: (i) compare aggregate size distribution in the surface and subsurface soils under a 10-yr-old rhizoma peanut and perennial weeds, (ii) determine NH4–N, NO3–N, organic C and N, PCM, PNM, MBC, MBN, POC, and PON concentrations in whole-soil and aggregates under peanut and weeds, and (iii) evaluate the effectiveness of long-term peanut productivity in improving soil aggregation, microbial activities, N mineralization, and C and N sequestration compared with weeds.
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MATERIALS AND METHODS
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Field Methods
The experiment was described in detail by Terrill et al. (1996). Briefly, rhizoma peanut (cv. Florigraze) plots (6 by 2 m) were established in April 1991 on a Norfolk loamy fine sand (fine-loamy, kaolinitic, thermic Typic Kandiudults) at Fort Valley State University, Agricultural Research Station farm in Fort Valley, GA. The plots were separated by additional 6- by 2-m plots of perennial weeds and alfalfa (Medicago sativa L.). Alfalfa died after 3 yr of establishment. For comparing soil properties between peanut and weeds, soil samples were collected only from these treatments. Thus, the experiment had two treatments: rhizoma peanut and perennial weeds. Although the experiment consisted of eight replications, only four replications containing intact growth of peanut and weeds were chosen for the study because the rest of the peanut plots were overgrown by weeds after 10 yr, even though herbicides were applied to control weeds. The soil had 750 g sand, 150 g silt, and 100 g clay kg-1 soil from the 0- to 30-cm depth, with 5.3 g kg-1 organic C, 465 mg kg-1 organic N, and pH 6.9 (1:2 soil:water ratio). Below the 30-cm depth, clay content increased to >300 g kg-1 soil.
Rhizomes of 5.25 m3 ha-1 were vegetatively transplanted in peanut plots arranged in a randomized complete block design. In the weed plots, perennial weeds [dominated by henbit and cut-leaf evening primrose] were allowed to grow naturally. Peanut plots were fertilized with 0 to 67 kg ha-1 P [from triple superphosphate, Ca(H2PO4)2, 48% P] and 146 to 224 kg ha-1 K [from muriate of potash, KCl, 60% K] fertilizers every year from 1991 to 2000 according to the soil test. No N fertilizer was applied. Weeds in peanut plots were controlled by applying sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 3.5 L a.i. ha-1 in mid to late summer every year. In the weed plots, no fertilizer or herbicide was applied. Both peanut and weeds were grown under natural rainfall and no irrigation was applied.
Rhizoma peanut biomass was harvested twice a year (July and October) from 1991 to 2000 when plants were 30 to 35 cm tall. A strip of plot (4 to 5 m long, 0.9 m wide) was cut at the center of the plot with a sickle-bar mower for biomass yield determination, after which the rest of the biomass was removed. In the weed plot, weeds were also mowed but biomass yield was neither determined nor removed. In September 2000, soil from an area of 900 cm2 and 15 cm depth containing roots and shoots of both rhizoma peanut and weeds was removed with a shovel and washed with water, after which roots and shoots were separated for determining total N concentration in each plant part. After final harvest of the aboveground biomass in early November 2000 when soil was relatively drier than in the previous months, intact soil cores from 0- to 90-cm depth were collected from three places within each plot using a push tube (120 cm long, 5.0 cm i.d.) attached to a hydraulic auger. Although a push tube of 120 cm long was used, intact core of soil samples was available only from the 0- to 90-cm depth while sample availability below 90 cm was irregular. Because of the small size (6 by 2 m) of plots, three holes to a depth of 90 cm per plot were considered adequate for collecting soil samples and analyzing for aggregate size distribution and associated C and N pools. Soil cores collected from the holes were separated into 0- to 15-, 15- to 30-, 30- to 60-, and 60- to 90-cm lengths, composited within a length to represent a particular depth, and stored at 4°C for 24 h.
Laboratory Analysis
Nitrogen concentration in root and shoot samples of rhizoma peanut and weeds was determined by the H2SO4–H2O2 method as described by Kuo et al. (1997). For determining aggregate-size distribution, soils were sieved according to Mendes et al. (1999) and Schutter and Dick (2002). After removing visible plant residues, field-moist soils (containing 150 g kg-1 water content in sandy loam at 0 to 15 cm to 240 g kg-1 in clay loam at the 60- to 90-cm depth) were passed through a 4.75-mm sieve and large particles retained in the sieve were gently crushed by hand to pass through it. Particles > 4.75 mm not crushed by hand contained mostly stones (<2%) and were discarded. The sieved soils were air-dried at 4°C for 7 d until they reached a gravimetric water content of 80 to 120 g kg-1, depending on clay content. This water content represented the moisture level at which soils can be sieved in finer sieves for aggregate-size separation according to our preliminary observations (data not shown). Drying the soil at 4°C reduces its impact on microbial communities and activities in aggregates (Mendes et al., 1999; Schutter and Dick, 2002). Aggregates were separated by placing 500 g of cold-dried soil fragments ( <4.75 mm) in a nest of sieves (20-cm diam.) containing 2.00-, 0.85-, 0.50-, and 0.25-mm sieves. These sieve sizes were chosen to separate soil aggregates of large, intermediate, and small sizes because microbial biomass and activities and N mineralization can vary significantly between aggregate-size classes (Mendes et al., 1999; Schutter and Dick, 2002). Dry sieving of soil was chosen to determine aggregate-size distribution and associated C and N pools over wet sieving because (i) dry sieving may reduce the disruption of physical habitat of microbial communities compared with wet sieving (Schutter and Dick, 2002), (ii) water-soluble C and N concentrations can be determined on aggregates separated by dry sieving which are not possible in wet sieving (Seech and Beauchamp, 1988; Beauchamp and Seech, 1990), and (iii) aggregates separated by dry sieving may represent more closely those in the field during the absence of rain or irrigation. Sieves were shaken horizontally by hand for 3 min at 
100 oscillations min-1, as preliminary experiments showed that this procedure adequately separated soil aggregates without destructing large aggregates (data not shown). Soils retained in sieves with size classes of 4.75- to 2.00-, 2.00- to 0.85-, 0.85- to 0.50-, 0.50- to 0.25-, and <0.25-mm diameter were weighed and stored at 4°C until chemical analysis were conducted. Mean-weight diameter of aggregates was calculated by summing the product of mean diameter of aggregates and proportion of soil in each aggregate-size class (Kemper and Rosenau, 1986).
Whole-soil and aggregate samples were air-dried prior to C and N analysis. Organic C concentration in whole-soil and aggregates was determined by the Walkley-Black method (Nelson and Sommers, 1996). Total N was determined by the Kjeldahl method (Bremner, 1996). The NH4 and NO3–N concentrations were determined by using steam distillation after extracting the soil with 2 M KCl (Mulvaney, 1996). Organic N was determined as the difference between total N and the sum of NH4 and NO3–N concentrations. Prior to the determination of microbiological properties, whole-soil and aggregates were moistened with water from 100 to 200 g kg-1 soil (60% water-filled pore space), depending on clay content, and stored at 21°C for 1 wk. The MBC in whole-soil and aggregates was determined by the fumigation-incubation method (Jenkinson and Powlson, 1976). Five to 10 g of whole-soil and aggregates (depending on the amount available) were fumigated with ethanol-free chloroform for 48 h and placed in a 1-L jar containing beakers with 10 mL of 0.5 M NaOH to trap evolved CO2 and 20 mL of water to maintain high humidity. At 10 d, CO2–C absorbed in NaOH was titrated with 0.05 M HCl from pH 8.3 to 3.7. The MBC was calculated by dividing the amount of CO2–C absorbed in NaOH by a factor of 0.41 (Voroney and Paul, 1984) without subtracting the values from nonfumigated control. For MBN, the fumigated-incubated sample at 10 d was extracted with 50 to 100 mL (1:10 soil/solution ratio) of 2 M KCl for 1 h and NH4–N concentration was determined by steam distillation. The MBN was calculated by the difference between NH4–N concentration in the fumigated-incubated sample at 10 d and in the sample just prior to fumigation and divided by a factor of 0.41 (Brookes et al., 1985). The PCM was determined by the amount of CO2–C evolved from 5 to 10 g of nonfumigated whole-soil and aggregates incubated in a 1-L jar at 21°C for 10 d. For PNM, the nonfumigated-incubated sample at 10 d was extracted with 50 to 100 mL (1:10 soil:solution ratio) of 2M KCl for 1 h and NH4 and NO3–N concentrations were analyzed by steam distillation. The PNM was calculated by the difference between the sum of NH4 and NO3–N concentrations in the nonfumigated-incubated sample at 10 d and in the sample immediately prior to incubation.
The POC and PON in whole-soil and aggregates were determined by the method described by Cambardella and Elliott (1992) with the following modifications: 5 to 10 g whole-soil and aggregates were dispersed with 15 to 30 mL (1:3 soil:solution ratio) of 5 g L-1 of sodium hexametaphosphate for 16 h and the solution was poured through a 0.05-mm sieve. After wet sieving and washing with deionized water, soil particles retained as fragments in the sieve were oven-dried at 50°C, weighed, ground using mortar and pestle and sieved to pass through a 0.25-mm sieve. The grinding and sieving continued until all particles passed through the sieve. The fine particles were thoroughly mixed and a subsample was used for determining organic C and N, using procedures as described above. Analysis of triplicate samples of fine particles showed POC values from 0.76 ± 0.05 (mean ± SD) g C kg-1 soil for the 4.75- to 2.00-mm aggregate-size class at the 0- to 15-cm depth to 0.15 ± 0.01 g C kg-1 soil for the <0.25-mm size class at 60 to 90 cm under rhizoma peanut and weeds. Similar ranges of standard deviation from the mean values of triplicate samples were observed for PON, suggesting that finely ground and thoroughly mixed fragments can be used as representative sample of the mixture for determining POC and PON. The difference between this method and that described by Cambardella and Elliott (1992) is that we used the particles retained in the 0.05-mm sieve (particles >0.05 mm) directly for determining POC and PON instead of calculating them as the difference between organic C and N concentrations in the whole-soil and in the particles that passed through the 0.05-mm sieve.
Particle size analysis in whole-soil and aggregates was determined by the pipette method after removing organic matter (Gee and Bauder, 1986). Because sand grains can be completely embedded into larger aggregates and clay can coat sand grains, we considered sand as a part of aggregation. As a result, data for C and N pools determined in soil aggregates were not normalized to sand-free basis.
Data Analysis
For analyzing data for aggregate size distribution, treatment (rhizoma peanut and perennial weeds) was considered as the main plot, soil depth as the split plot, and aggregate-size class as the split-split plot treatment. Data were analyzed using the MIXED procedure of SAS after testing for homogeneity of variance (Littell et al., 1996). Means were separated by using the least square means test when the interaction was significant. For comparing C and N pools in whole-soil and aggregates, whole-soil was also considered as an another aggregate-size class and data were analyzed as above. When soil depth x aggregate-size class interaction was significant, means within a treatment were separated by the least square means test because each treatment had different management practice. When treatment x soil depth interaction was significant, means for C and N pools in whole-soil instead of means averaged across aggregate-size class were separated by the least square means test because mean separation in whole-soil only gave meaningful results. Statistical significance was evaluated at P 
0.05.
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RESULTS
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Soil Aggregate-Size Distribution
Aggregate-size class and its interaction with soil depth were highly significant for the amount of soil present in aggregates under rhizoma peanut and perennial weeds while mean-weight diameter was significant for soil depth (Tables 1 and 2). While the amount of soil present in aggregates decreased with decrease in aggregate-size class at all depths, it varied with depth for a particular size class (Table 2). For example, under both treatments, the amount of soil present was greater at 15 to 60 cm than at the 0- to 15-cm depth for 4.75- to 2.00-mm size class but was greater at 30 to 60 than at 0 to 15 cm for the 2.00- to 0.85-mm size class. In contrast, for the <0.50-mm size class, the amount of soil present was greater at 0 to 15 than at 30 to 60 cm. Mean-weight diameter of aggregates was greater at 15 to 60 than at 0 to 15 cm.
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Table 1. Analysis of variance for soil aggregate and C and N concentrations. Whole-soil data was excluded from soil aggregate data in the analysis for the amount of soil present in aggregate and its mean-weight diameter (MWD) but was included in the analysis for C and N concentrations.
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Table 2. Influence of soil depth and aggregate-size class on the amount of soil present in aggregates and mean-weight diameter (MWD) under rhizoma peanut and perennial weeds.
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Sand Content
Soil depth and aggregate-size class were significant for sand content in whole-soil and aggregates under rhizoma peanut and perennial weeds (P 0.05). As a result, sand content in whole-soil and aggregates decreased with increased depth but increased with decreased aggregate-size class under both peanut and weeds (Table 3). The variation in sand content with depth and aggregate-size class is expected to influence aggregation in the surface and subsurface soils.
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Table 3. Influence of soil depth and aggregate-size class on sand content in aggregates and whole-soil under rhizoma peanut and peren-nial weeds.
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Soil Organic Carbon and Nitrogen
Treatment and its interaction with soil depth or aggregate-size class did not influence soil organic C and N concentrations, but depth and depth x aggregate-size class interaction were significant (Tables 1 and 4). While organic C and N concentrations in whole-soil and aggregates decreased with increasing depth, concentrations in aggregates at 0 to 15 cm increased with decreasing size class under both rhizoma peanut and weeds (Table 4). At 0 to 15 cm, organic C and N were greater in the <0.25- than in the 4.75- to 0.50-mm size class, and greater in the 0.85- to 0.50- than in the 4.75- to 2.00-mm size class. Organic C in whole-soil was similar to that in the 4.75- to 0.85-mm size class under peanut and to that in the 2.00- to 0.50-mm size class under weeds. Similarly, organic N in whole-soil was similar to that in the 2.00- to 0.25-mm size class under peanut and similar to that in the <2.00-mm size class under weeds.
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Table 4. Influence of soil depth and aggregate-size class on organic C and N concentrations in aggregates and whole-soil under rhizoma peanut and perennial weeds.
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Soil Inorganic Nitrogen
Treatment x soil depth interaction was significant for NH4–N concentration but depth and depth x aggregate-size class interaction were significant for NO3–N concentration (Tables 1 and 5). The NH4–N in whole-soil at 0 to 15 cm was greater under rhizoma peanut than under weeds (Fig. 1A)
. While NO3–N in whole-soil under peanut decreased with increasing depth, NO3–N at 0 to 15 cm was greater in aggregate-size class >0.85 mm than in <0.50 mm (Table 5). Under weeds, NO3–N varied with aggregate-size class and depth.
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Table 5. Influence of soil depth and aggregate-size class on NO3–N concentration in aggregates and whole-soil under rhizoma peanut and perennial weeds.
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Fig. 1. Concentrations of (A) NH4–N, (B) potential C mineralization, and (C) microbial biomass C in whole-soil from the 0- to 90-cm depth under rhizoma peanut and weeds. Bars followed by different letters are significantly different between peanut and weeds and between soil depths at P 0.05 by the least square means test.
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Potential Carbon and Nitrogen Mineralization
Soil depth and treatment x depth interaction were significant for PCM and depth was significant for PNM (Table 1). Aggregate-size class and its interaction with depth were not significant for PCM and PNM. The PCM in whole-soil at 30 to 90 cm was greater under rhizoma peanut than under weeds (Fig. 1B). Both PCM and PNM in whole-soil under peanut and weeds decreased with increasing depth. The PCM ranged from 225 mg kg-1 soil at 0 to 15 cm to 50 mg kg-1 at 60 to 90 cm, and PNM ranged from 23 mg kg-1 at 0 to 15 cm to 10 mg kg-1 at 60 to 90 cm.
Microbial Biomass Carbon and Nitrogen
Soil depth, treatment x depth, and depth x aggregate-size class interactions were significant for MBC, and depth and depth x aggregate-size class interaction were significant for MBN (Tables 1 and 6). The MBC at 0 to 30 cm was greater under rhizoma peanut than under weeds (Fig. 1C). At 0 to 15 cm, MBC was greater in the <0.25-mm than in the 4.75- to 0.85-mm aggregate-size class and in whole-soil under both peanut and weeds, but at 60 to 90 cm, MBC was greater in the 4.75- to 0.85-mm than in the <0.50-mm size class and in whole-soil under peanut (Table 6). Similarly, MBN was greater in the <0.25-mm than in the 4.75- to 0.85-mm size class at 0 to 15 cm under peanut and weeds. The MBN in whole-soil was similar to that in the 2.00- to 0.25-mm size class.
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Table 6. Influence of soil depth and aggregate-size class on microbial biomass C (MBC) and N (MBN) concentrations in aggregates and whole-soil under rhizoma peanut and perennial weeds.
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Particulate Organic Carbon and Nitrogen
Soil depth and treatment x depth interaction were significant for the amount of particles as fragments retained in the 0.05-mm sieve after wet sieving of soil dispersed with sodium hexametaphosphate (particles > 0.05 mm) and PON (Table 1). Similarly, depth x aggregate-size class interaction was significant for POC and PON. Particles > 0.05 mm in whole-soil at 15- to 30 and 60 to 90 cm were greater under rhizoma peanut than under weeds (Fig. 2A) . Similarly, PON at 0 to 15 cm was greater under peanut than under weeds (Fig. 2B). At 0 to 15 cm, POC and PON were greater in the <0.25-mm than in the 4.75- to 0.50-mm aggregate-size class and in whole-soil under peanut and weeds, except for POC in whole-soil under peanut (Table 7).
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Fig. 2. (A) Amount of particles as fragments retained in a 0.05-mm sieve used for determining particulate organic C and N after wet sieving of whole-soil extracted with sodium hexametaphosphate (particles > 0.05 mm), and (B) concentration of particulate organic N in whole-soil from the 0- to 90-cm depth under rhizoma peanut and weeds. Bars followed by different letters are significantly different between peanut and weeds and between soil depths at P 0.05 by the least square means test.
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Table 7. Influence of soil depth and aggregate-size class on particulate organic C (POC) and N (PON) concentrations in aggregates and whole-soil under rhizoma peanut and perennial weeds.
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DISCUSSION
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Although wet sieving is used to determine aggregate-size distribution to represent aggregation in the field due to rain drop impact on dry soil causing slaking and surface crusting, dry sieving of moist soil for determining microbial biomass and activities in aggregates is getting increasing attention (Mendes et al., 1999; Schutter and Dick, 2002). This is because dry sieving may provide some advantages over wet sieving, such as less destruction of physical habitat of microbial communities as suggested by Schutter and Dick (2002) and determinations of water soluble C and N in aggregates separated by dry sieving which are not possible in aggregates separated by wet sieving. Wet sieving may increase the destruction of physical habitat of microbial communities in aggregates because it ruptures aggregates due to ion hydration and osmotic swelling forces or to solubility of binding agents in water (Kemper and Rosenau, 1986; Perfect et al., 1990). Dry sieving also ruptures aggregates due to abrasive action of sieves so that average aggregate size is reduced to somewhat below that in field (Kemper and Rosenau, 1986); however, it may not be as destructive as wet sieving in maintaining the physical habitat of microbial communities in aggregates. The choice of the method in separating aggregates may influence aggregate-size distribution, physical habitat of microbial communities, and the levels of C and N pools in aggregates.
The increased amount of soil present but decreased levels of C and N pools with increasing aggregate-size class at 0 to 15 cm (Tables 2, 4, 6, and 7) and nonsignificant difference in C and N pools between size classes at 15 to 90 cm under rhizoma peanut and perennial weeds (Tables 4, 6, and 7) suggests that enmeshing action of roots probably play a greater role in aggregation than aggregate-associated organic matter and microbial biomass in undisturbed soil under forages, as suggested by Tisdall and Oades (1979) and Jastrow et al. (1998). Although root distribution under perennial weeds was not measured, a deep distribution of rhizoma peanut roots (>60-cm depth) has been reported (Terrill et al., 2000). Because of the limited knowledge, the relationships between root distribution, soil aggregation, and associated C and N pools under forages needs to be further explored. The greater amount of soil present in the 4.75- to 0.85-mm aggregate-size class or mean-weight diameter of aggregates at 15 to 60 than at 0 to 15 cm under rhizoma peanut and perennial weeds (Table 2) suggests that aggregation in the subsurface soil may have improved by increased cohesive tension of silt and clay particles to form large aggregates compared with sand particles, because sand content in whole-soil and aggregates decreased or silt and clay content increased with increased soil depth (Table 3). Increased clay content increases proportion of macroaggregates (4.75–1.00 mm) in the soil (Franzluebbers et al., 2000). Increased aggregation as determined by dry sieving in the subsurface soil may improve water infiltration capacity and root growth (Kemper and Rosenau, 1986). It may also reduce soil erosion due to wind if the subsurface soil is exposed, such as in sloping or denuded land.
The relationship between soil aggregation and associated C and N pools is complex. In cover cropped and fallow soils under conventional tillage, Mendes et al. (1999) and Schutter and Dick (2002) observed greater C and N pools in intermediate size (1.00–0.25 mm) aggregates than in macro- or microaggregates obtained by dry sieving. While Schutter and Dick (2002) observed increased amount of soil present in aggregates with increasing aggregate-size class, Mendes et al. (1999) did not find significant difference in the amount of soil present between aggregate-size classes. In the undisturbed soil at the 0- to 15-cm depth under forages, we observed an increased amount of soil present but decreased levels of C and N pools with increasing aggregate-size class (Tables 2, 4, 6, and 7). Probably, tillage and amount and placement of plant biomass in the soil may have influenced the relationship between soil aggregation and C and N pools found in our study and those observed by Mendes et al. (1999) and Schutter and Dick (2002). While soils under forages in our study were not tilled and aboveground plant biomass (left over after harvest in rhizoma peanut or all in weeds) were placed at the surface, soils under cover crops and weeds in their studies were tilled and plant biomass was incorporated into greater depth. Tillage can disintegrate macroaggregates into microaggregates and result in an even distribution of plant residue (Molope et al., 1987, Six et al., 1998) and microbial communities (Petersen et al., 1997) among soil aggregates. In the undisturbed soil of perennial forage production systems, reduced incorporation and distribution of plant residue probably increased the level of substrate availability for microbes at the soil surface and decreased with increasing depths. While this may flourish microbial activities at the surface compared with the subsurface soil, uneven distribution of plant residue among aggregates due to lack of tillage may have reduced the levels of C and N pools in macroaggregates compared with microaggregates under forages. Air-drying of soil also may have increased cohesive tension between microaggregates (Kemper and Rosenau, 1986), thereby increasing the amount of soil present with increasing aggregate-size class. Another possible reason may be that energy imparted with dry sieving was not sufficient to break the bonds holding the microaggregates together.
Although organic C and N levels in the whole-soil and aggregates obtained by dry sieving at 0 to 15 cm in this study were lower than in the soils used by Mendes et al. (1999), Ndiaye et al. (2000), and Schutter and Dick (2002), MBC values of 248 to 523 mg C kg-1 and PCM values of 173 to 225 mg C kg-1 in whole-soil and aggregates obtained in this study were similar to or greater than their reported MBC values of 86 to 400 mg C kg-1 and PCM values of 20 to 131 mg C kg-1 soil. These differences were probably due to variation in tillage practices, amount, type, and placement of plant biomass, soil type, and environmental conditions between the locations.
Our results for greater MBC and MBN in microaggregates than in macroaggregates in dry-sieved soils at 0 to 15 cm (Table 6) were similar to those observed by Seech and Beauchamp (1988) and Mendes and Bottomley (1998). Seech and Beauchamp (1988) found greater microbial biomass and activities in microaggregates than in macroaggregates when aggregates were separated by dry sieving but found greater activities in macroaggregates when separated by wet sieving. As a result, they suggested that wet sieving probably removed a greater concentration of water soluble C in microaggregates than in macroaggregates, thereby decreasing microbial biomass and activity in microaggregates separated by wet sieving. The reasons for greater MBC level in the 4.75- to 0.85-mm size class at 60 to 90 cm under peanut were not known but probably a result of peanut root or soil textural effect.
In contrast to other C and N pools, greater NO3–N concentration in the 4.75- to 0.85-mm than in the <0.50-mm aggregate-size class at 0 to 15 cm under peanut and at 15 to 30 cm under weeds (Table 5) indicates that macroaggregates contained a greater level of available N. The reasons for this are not clear, but it may be possible that macroaggregate surfaces have greater levels of N pools that are readily converted into available forms.
The greater NH4–N and PON concentrations in whole-soil at 0 to 15 cm under rhizoma peanut than under weeds (Fig. 1A and 2B) indicates that peanut may have increased labile soil N pools by fixing atmospheric N in roots and shoots and by increasing N input in soil through leaf and branch fall and dying root residue. Although N input from peanut and weed biomass from 1991 to 1999 were not known, analysis of biomass samples taken in September 2000 showed that N concentration in rhizoma peanut roots to a depth of 15 cm was 15.1 g kg-1 and N accumulation was 94 kg ha-1 compared with 8.5 g kg-1 and 36 kg ha-1, respectively, in weeds. Because soil samples were taken only from 0 to 15 cm for root biomass study, it is expected that peanut roots will have greater N input than weed roots at 15 to 90 cm and beyond, although labile N pools at this depth were not different between peanut and weeds. Similarly, N concentration in aboveground biomass of peanut was 28.2 g kg-1 and N accumulation was 150 kg ha-1 compared with 11.8 g kg-1 and 34 kg ha-1, respectively, in weeds. Although peanut biomass was removed every year but weed biomass was not, N input from peanut root biomass (without considering inputs from branch and leaf fall) may still be greater than N input from above and belowground biomass of weeds in the soil if the above trend continued from 1991 to 1999 when forages were established. Higher concentration and accumulation of N in legumes can increase soil N concentration (Kuo et al., 1997; Sainju et al., 2000). Although seasonal distribution of rainfall may influence inorganic N level in soil, increased PON level under rhizoma peanut treatment suggests that peanut will enrich a greater level of available N than weeds in the soil. Similarly, greater PCM at 30 to 90 cm, MBC at 0 to 30 cm, and particles >0.05 mm at 15 to 30 and 60 to 90 cm (Fig. 1B, 1C, and 2A) under peanut than under weeds indicates that peanut roots also may stimulate microbial biomass and activity in the surface and subsurface soil. Nonsignificant difference in soil organic C and N between peanut and weeds suggests that rhizoma peanut did not increase C and N sequestration in the soil compared with perennial weeds, even after 10 yr.
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SUMMARY AND CONCLUSIONS
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Long-term productivity (10 yr) of rhizoma peanut may increase microbial biomass and activities and labile N pools in the surface and subsurface soils compared with perennial weeds, although it did not increase C and N sequestration in the soil. Aggregation in the subsurface soil improved relative to the surface soil when silt and clay contents increased with increasing depth. However, improved soil aggregation was not followed by increased C and N pools in aggregates, rather the pools in whole-soil and aggregates decreased with increasing soil depth or increased with decreasing aggregate-size class. In contrast, NO3–N concentration was greater in large (4.75–0.85 mm) than in small (<0.50 mm) aggregates. While rhizoma peanut may improve soil quality and fertility compared with perennial weeds, improved aggregation in the subsurface compared with surface soil under peanut and weeds may improve water infiltration capacity and root growth and reduce soil erosion when subsurface soil is exposed. In the undisturbed soil under forages, microbial biomass can flourish as substrate availability increased with decreased aggregate-size class or decreased soil depth but improved aggregation may also increase available N. Rhizoma peanut may improve soil quality and fertility by increasing microbial activities and enriching soil N compared with perennial weeds.
Received for publication October 10, 2001.
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ABSTRACT
INTRODUCTION
