L. Brussaard

Biological effects of plant residues with contrasting chemical compositions under humid tropical conditions decomposition and nutrient release.

Decomposition and nutrient release patterns of prunings of three woody agroforestry plant species (Acioa barteri, Gliricidia sepium and Leucaena leucocephala), maize (Zea mays) stover and rice (Oryza sativa) straw, were investigated under field conditions in the humid tropics, using litterbags of three mesh sizes (0.5, 2 and 7 mm) which allowed differential access of soil fauna. The decomposition rate constants ranged from 0.01 to 0.26 week−1, decreasing in the following order; Gliricidia prunings >Leucaena prunings > rice straw > maize stover >Acioa prunings. Negative correlations were observed between decomposition rate constants and C:N ratio (P < 0.004), percent lignin (P < 0.014) and polyphenol content (P < 0.053) of plant residues. A positive correlation was observed between decomposition rate constant and mesh-size of litterbag (P < 0.057). These results indicate that both the chemical composition of plant residues and nature of the decomposer played an important role in plant residue decomposition. Nutrient release differed with quality of plant residues and litterbag mesh-size. Total N, P, Ca and Mg contents of plant residues decreased with time for Gliricidia and Leucaena prunings, maize stover, and rice straw, and increased with time for Acioa prunings. There was some indication of N immobilization in maize stover and rice straw; P immobilization in Leucaena prunings and rice straw; and Ca immobilization in maize stover, rice straw and Gliricidia and Leucaena prunings. Acioa prunings immobilized N, P, Ca and Mg. All plant residues released K rapidly. Nutrient release increased with increasing mesh-size of litterbags, suggesting that soil faunal activities enhanced nutrient mobilization.

Relationships between habitable pore space, soil biota and mineralization rates in grassland soils.

Abstract The hypotheses that the accessible soil pore volume determines the biomass of bacteria and their grazers, and that the activity of bacteria and the mineralization rates of C and N are affected by grazing pressure on bacteria were tested. We determined the biomass of bacteria, fungi, protozoa and nematodes, the pore-size distribution, and the potential mineralization rates of C and N in grassland soils with different textures. Bacteria constituted by far the largest biomass pool. Fungi, protozoa and nematodes together comprised only 10% of the total biomass. It was found that in loams and clays, most pores had diameters Bacterial activity [measured as the frequency of dividing-divided cells (FDDC); the number of viable cells; and the amount of CO 2 produced per cell] were not affected by grazing intensity. The amount of N mineralized bacterium −1 , however, was much higher in soils with a high grazing pressure of bacterivorous nematodes and flagellates than in soils with a low grazing pressure of these groups. This indicates that grazing of bacteria by bacterivorous nematodes and flagellates may considerably increase N mineralization. No relationship was found between the grazing pressure of amoebae and the amount of N mineralized bacterium −1 .

Relationships between soil texture, physical protection of organic matter, soil biota, and C and N mineralization in grassland soils.

Abstract The effect of soil type on carbon (C) and nitrogen (N) mineralization rates in grassland soils was investigated along with the physical and biological soil characteristics that may have caused the observed differences in mineralization rates between soil types. The percentage of mineralized organic N was higher in sandy soils than in loams and clays; this was not observed for C. In loams and clays small pores constituted a higher percentage of the total pore space than in sandy soils. Two mechanisms of physical protection of organic N were distinguished. In clay soils physical protection of organic material by its location in small pores was the main mechanism. In sandy soils, however, organic material was protected by its association with clay particles. In loams both mechanisms played a role. The protected organic material associated with clay particles consisted of amorphous undefined material that did not stain with acridine orange, indicating a high degree of decomposition, while the non-protected organic material present in the sand fraction consisted of plant debris that stained intensely with acridine orange. Physically protected organic matter had a lower C/N ratio than organic matter that was not physically protected. Grazing pressure on bacteria by bacterivorous nematodes was higher in sandy soils than in loams and clays. This coincided with a higher N mineralization rate per bacterium. The C/N ratio of the microbial biomass was higher in sandy soils than in loams and clays and was positively correlated with the N mineralization rate per unit of microbial biomass N. This is in agreement with the concepts of food webs that N mineralization is positively correlated with the C/N ratio of the consumer (bacteria) for a given sol|C/N ratio of the substrate (organic matter). It is not yet clear which of the factors investigated are the most important in determining N mineralization rates in grassland soils.

Simulation of nitrogen mineralization in the belowground food webs of two winter wheat fields.

Food webs in conventional (high-input) and integrated (reduced-input) farming systems were simulated to estimate the contribution of soil microbes and soil fauna to nitrogen mineralization during the growing season. Microbes accounted for approximately 95% of the biomass and 70% of total nitrogen mineralization in both management practices. Among the soil fauna, amoebae and bacterivorous nematodes were the most important contributors to nitrogen mineralization. The contribution of nematodes showed more temporal and spatial variability than the contribution of amoebae. The model calculated nitrogen mineralization rates close to the observed rates for both fields and depth layers. In the integrated plot there were relatively high rates of mineralization in the 0-10 cm layer compared with the 10-25 cm layer, whereas in the conventional plot no differences were observed between depth layers (...)

Calculation of nitrogen mineralization in soil food webs

In agricultural practices in which the use of inorganic fertilizer is being reduced in favour of the use of organic manure, the availability of nitrogen (N) in soil for plant growth depends increasingly on N mineralization. In simulation models, N mineralization is frequently described in relation to the decomposition of organic matter, making a distinction in the quality of the chemical components available as substrate for soil microbes. A different way to model N mineralization is to derive N mineralization from the trophic interactions among the groups of organisms constituting the soil food web. In the present study a food web model was applied to a set of food webs from different sites and from different arable farming systems. The results showed that the model could simulate N mineralization rates close to the rates obtained from in situ measurements, from nitrogen budget analyses, or from a decomposition based model. The outcome of the model suggested that the contribution of the various groups of organisms to N mineralization varied strongly among the different sites and farming systems.

Microbial numbers and activity in dried and rewetted arable soil under integrated and conventional management

Abstract During an 8-week microplot experiment, effects of moisture regime and farm management on microbial numbers and activity were studied. Under integrated management (reduced input farming), bacterial numbers, O 2 consumption and N mineralization, respectively, were 1.6, 2.1 and 1.8 times higher than under conventional management (high input farming). These differences may be attributed to 1.3 and 1.4 times higher contents of organic matter and total N in the integrated microplots. One month of drying from a water potential of −0.03 to −0.12 MPa, and subsequent rewetting to −0.01 MPa, did not affect bacterial numbers significantly. However, the relatively small decrease in water potential caused a significant decrease in O 2 consumption and N mineralization. After rewetting, respiration increased from 1.3 to 1.5 fold, and N mineralization from 3 to 5 fold. Concurrently, the frequency of dividing-divided cells (FDDC) increased from 10 to 16% in the conventional and to 23% in the integrated microplots. This suggests that the FDDC, which is determined by direct microscopy and requires no incubation, can be used as an index of in situ bacterial growth rate in soil. For marine bacteria, mathematical relationships have been established between specific growth rate (μ) and FDDC. If it is assumed that these relationships are also valid for soil bacteria, FDDCs of 16 and 23%, respectively, may indicate specific growth rates of about 1 and 2 day −1 . Bacterial production rates based on FDDC (8.5–45 μg C g −1 day −1 ) were 3–8 times higher than those based on O 2 consumption rates determined by 2-week incubations. Uncertainties of the methods are discussed.

Biological effects of plant residues with contrasting chemical compositions under humid tropical conditions: Effects on soil fauna

Abstract Effects of application of five types of plant residues [Acioa barteri, Gliricidia sepium and Leucaena leucocephala prunings, maize (Zea mays) stover and rice (Oryza sativa) straw] as mulch on soil fauna were examined under field conditions in the humid tropics in 1990 and 1991. Earthworm mean population over 2 years was higher under any type of plant residues by 41% compared to control. Leucaena prunings supported the highest earthworm population. Mulched plots also showed 177% higher mean termite population over 2 years than control. Highest termite population was observed in plots mulched with Acioa prunings followed by maize stover > rice straw >Leucaena prunings >Gliricidia prunings. The mean ant populations were 36% higher with Leucaena and Gliricidia prunings, and were not affected by Acioa prunings, maize stover and rice straw as compared to control. Millipede populations were not significantly affected by mulching. Earthworm populations were negatively correlated with the ratio of lignin : N of plant residues. Ant populations were significantly related to the N content of plant residues (R2 = 0.87 and 0.84 for 1990 and 1991 respectively). The results imply that chemical plant composition, particularly N and lignin contents, play a critical role in faunal abundance in the soil through their effect on palatability and decomposibility. Indirect microclimatic and mulching effects may also be important.

Effects of chemical composition on N, Ca, and Mg release during incubation of leaves from selected agroforestry and fallow plant species

Nitrogen, Ca and Mg release from leaves of ten selected plant residues with varying chemical compositions was studied under laboratory conditions. Three patterns of N-release were observed over a seven week incubation period: (a)Gliricidia sepium, Leucaena leucocephala, Mucuna pruriens andCentrosema pubescens leaves showed rapid N release, (b)Acioa barteri andDialium guineense leaves immobilized N, and (c)Alchornea cordifolia, Anthonata macrophylla, Cassia siamea andPterocarpus santalinoides leaves initially showed N immobilization which gradually changes to net mineralization after about four weeks of incubation. Nitrogen mineralization rate constant (k) ranged from −0.0018 (A. barteri) to 0.0064 day−1 (G. sepium). Statistical analysis of data showed that N mineralization rate constants are significantly correlated with initial N, polyphenol and lignin contents of leaves. Nitrogen release increased with increasing N content and decreased with increasing contents of polyphenols and lignin.Addition of leaves from all species significantly increased soil exchangeable Ca and Mg levels.L. leucocephala, G. sepium, C. pubescens andM. pruriens showed relatively high Ca and Mg release rates. Calcium release rate was related to N release rate rather than to initial Ca content.

An index for assessing the quality of plant residues and evaluating their effects on soil and crop in the (sub-) humid tropics

Abstract An equation was developed for calculating a plant residue quality index (PRQI) in the (sub-)humid tropics using the C/N ratio and lignin and polyphenol concentration of plant residues. Among 18 plant species tested, there was a large variation in PRQI. The PRQI was correlated with the decomposition rate of plant residues, soil microclimate, soil fauna density and maize crop performance in the field. Soil moisture and termite density increased with decreases in PRQI, whereas decomposition rate constants of plant residues, soil temperature and ant density increased with increase in PRQI. Improvement in crop performance, such as grain yield, by plant residue mulching was lowest in the case of intermediate PRQI. It is concluded that PRQI can be used for selecting plant residues and projecting their agronomic value in the tropics.

Mulching effect of plant residues with chemically contrasting compositions on maize growth and nutrients accumulation

Effects of application of prunings of three woody species (Acioa barteri, Gliricidia sepium and Leucaena leucocephala), maize (Zea mays L.) stover and rice (Oryza sativa L.) straw as mulch on maize were studied on an Alfisol in southern Nigeria in 1990 and 1991. Maize dry matter and grain yield were higher with applications of plant residues and N fertilizer in both years. Addition of Leucaena prunings gave the highest maize grain yield in both years. Compared to the 1990 results, Acioa showed the least grain yield decline among the mulch treatments in 1991. Nutrient uptake was enhanced by applications of plant residues. Leucaena prunings had the highest effect in both years and increased the mean N, P, and Mg uptake by 96%, 84%, and 50%, respectively, over the control. Addition of Acioa prunings increased K and Ca uptake by 59% and 92%, respectively, over the control. ‘High quality’ (low C/N ratio and lignin level) plant residues enhance crop performance through direct nutritional contributions, whereas ‘low quality’ (high C/N ratio and lignin level) plant residues do so through mulching effects on the microclimate. ‘Intermediate quality’ plant residues have no clear effects on crop performance.