Biology and Fertility of Soils

Wekerom forest shows a high nitrogen (N) load, and the first signs of N saturation. This characterization is based on the high N content of the needles, the high nitrate-N (NO3-N) mobilization and low cation mobilization from the organic horizon. The N cycle in this forest has been transformed into an „open flow” system, in which the ammonium-N, deposited in large quantities from the atmosphere, is transformed into NO3-N, which is leached into the groundwater. Decomposition of deeper organic layers, such as the fragmented litter and humus layers, is thought to provide additional NH4-N, which explains the high NO3-N output. Together with a reduction in the number and vitality of the pine trees, there is an increase in the number of nitrophilous plants, such as Deschampsia flexuosa and Rumex acetosella. The ectomycorrhizal and litter-decaying fungi are specific, N-resistant species. Soil fauna are classified as common inhabitants of dry, acid, nutrient poor forests.

 Marigolds (genus Tagetes) suppress populations of soil endopathogenic nematodes such as Pratylenchus penetrans and Meloidogyne species. Nematode suppression by marigolds is thought to be due to thiophenes, heterocyclic sulfur-containing molecules abundant in this plant. When activated, thiophenes such as α-terthienyl produce oxygen radicals. If marigold roots release such a powerful biocidal agent and it is activated in soil, microbial populations in the marigold rhizosphere should be substantially perturbed. We made various measurements of microbial population size and activity in soils that had been cropped to marigolds (Crackerjack, Creole) in the field and in the greenhouse, and compared these with bare soil and soil cropped to rye (Secale cereale L.). Total extractable microbial biomass (measured by the fumigation extraction method), total bacteria (measured by epifluorescence microscopy on 5-(4,6-dichlorotriazine-2-γl) aminofluorescein-stained preparations), heterotrophic bacteria (measured by plate count on various media), and nitrite-oxidizing bacteria (measured by the most-probable-number method) were not significantly different in any of the treatments. Residues of 14C-labelled rye were mineralized slightly more rapidly in rye-cropped soil than in the other treatments, which were comparable. The rates of die-back of introduced cells of the bacteria Escherichia coli and Rhodococcus TE1 were similar in marigold-cropped and control soils, suggesting that there was not a noteworthy accumulation of biocidal agents in soils cropped to marigolds. We conclude that marigolds do not cause a general depression in the numbers of microorganisms in soils, and that nematode control by this plant may not be due to the release of a biocidal agent into the soil.

The modification of bentonite clays by cetyltrimethylammonium bromide (CTMAB) surfactant via cation-exchange produces materials (“organo-clays”) with an increased capacity for sorbing organic compounds such as pesticides. The sorption from solutions of two nonionic pesticides, methyl-parathion and carbaryl, by an organo-bentonite has been investigated. The pesticides are partitioned into the surfactant. The distribution coefficients, K ss, show a strong dependence on surfactant loading of the bentonite. The surfactant configuration at the clay surface has a marked influence on the effective volume and density of the bound surfactant. At low surfactant loadings, the K ss values increased, reached a maximum, and then decreased as the extent of loading increased. At low loading levels, the surfactant appears to form a monolayer (organic film) that effectively adsorbs the pesticides, resulting in very high K ss values. At high loadings, the sorbed surfactant appears to form a bulk-like medium that behaves essentially as a distribution phase. As a result, the K ss values decreased appreciably, and became less dependent on the CTMAB loading. Moreover, when the surfactant concentration in water was greater than the critical micelle concentration, the surfactant uptake on the clay reaches a plateau and an increasing fraction of the micelles remain in solution, together with the pesticides which bound to them. The competition for the pesticides between the aqueous micelles and the sorbed surfactant leads to a decrease in distribution coefficients.

Cover crop (CC) decomposition and subsequent release of nitrogen (N) are highly influenced by residue water potential (ψ) and temperature (T). To evaluate how carbon (C) and N mineralization from surface-applied CC residues responds to changes in ψ and T, a controlled microcosm experiment was conducted for 150 days with three CC residues (early-killed cereal rye (Secale cereale L.), late-killed cereal rye, late-killed crimson clover (Trifolium incarnatum L.), and a soil-alone control) under different ψ (−0.03, −1.5, −5, and −10 MPa) and T (15, 25, and 35 °C) conditions. Headspace gas was sampled periodically to determine carbon dioxide (CO2) and nitrous oxide (N2O) emissions. Soil inorganic N was determined by destructive sampling at 15, 30, 60, 100, and 150 days. Temporal dynamics in C and N mineralization from surface-applied CC residues were adequately described by first-order rate kinetic models. Early-killed rye and crimson clover (low C:N) residues decomposed quickly and mineralized N, whereas, late-killed rye residue (high fiber content and C:N) immobilized N. The normalized values of C and N mineralized from surface-applied CC residues increased exponentially with increasing ψ from −10.0 to −0.03 MPa. Increasing T from 15 to 35 °C further amplified the effect of ψ, suggesting a strong interactive effect of ψ and T on C and N mineralization from CC residues. Mathematical equations were developed to describe these interactive effects. Existing computer simulation models (e.g., CERES-N) could be improved by integrating these equations to simulate the effect of environmental conditions on surface-applied CC residue decomposition and N mineralization.

The effects of soil compaction on earthworm ( Aporrectodea caliginosa nocturna) activity were studied using pot experiments. Two compaction pressures were used when packing the pots; loose soil was packed by applying a pressure of 96 kPa, and compact soil was packed using a compaction pressure of 386 kPa. “Split pots” which contained both loose and compact soil were also used. In split pots peripheral burrows (those next to the pot wall), were generally longer in the loose half of the pot than they were in the compact half. This implies that earthworms preferred the loose soil. An important finding was that peripheral burrows in the split pots were longer than those made in pots packed uniformly with either loose or compact soil. Our data suggest that the increase in activity in split pots is a response to spatial variation in soil compaction. In the split pots the earthworms tended to avoid the compact half, and we discuss how the avoidance response in the split pots may have increased the proportion of burrows in these pots that were peripheral.

Soil gross nitrogen (N) transformation rates are highly sensitive to land use change. However, understanding the effect of land use change on internal N cycling patterns and its underlying mechanisms in tropical soils remains elusive. Here, four typical land uses including forest (> 400 years), eucalyptus (15 years), rubber (35 years), and paddy field (40 years) plantations in tropical region of China were investigated. The technique of 15N tracing was used to quantify soil gross N transformation rates. We also measured soil biochemical properties as well as carbon (C) and N fractions to evaluate the controls on any changes in soil N cycling processes. We found that converting natural tropical forests to managed ecosystems shifts the soil N dynamics from nitrate-dominated N forms towards ammonium-dominated N forms, suggesting that managed ecosystems becoming conservative (i.e., lower ratio of autotrophic nitrification (ONH4) to ammonium immobilization (INH4) and nitrous oxide (N2O) emissions and higher nitrate immobilization) than the natural tropical forest. The higher tendency of N loss (i.e., higher ONH4/INH4 and N2O emissions) of the natural tropical forest was mainly due to the higher concentrations of soil total N and hydrolysable ammonium N and microbial biomass, which stimulated ONH4. Lower microbial biomass, hydrolysable ammonium N, particulate organic C, and gross N mineralization, however, significantly decreased ONH4 in managed ecosystems. Our study also showed a pivotal role of soil C and N fractions in controlling soil heterotrophic nitrification, which enhanced significantly with decreasing amino sugar N, amino acid N, dissolved organic C, easily oxidizable organic C, and light fraction organic C. Our findings highlighted the pivotal role of soil C and N fractions in regulating soil N cycling under future land use changes.

A study was conducted in a sweet pepper-maize-rice cropping system in six farmers’ fields in Batac, Ilocos Norte, the Philippines, to determine the optimum P fertilizer rate for sweet pepper that will benefit the succeeding crops, maximize system-level productivity and profitability, and reduce the excessive accumulation of P in the soil. Single super phosphate was applied to sweet pepper at rates of 0, 28, 56, 84, 112, and 140 kg P ha−1 and the succeeding crops were grown without P fertilization. Maize residue was incorporated into the soil at puddling of soil for rice. Phosphorus fertilization at 56 kg P ha−1 and above had a residual effect on maize and rice. A reduction in the P applied to sweet pepper from 140 to 84 kg P ha−1 reduced extractable P in the soil at rice harvest from 52 to 29 kg P ha−1. Phosphorus applied at 111 kg P ha−1 to sweet pepper was optimum for maximum productivity and economic returns of the sweet pepper-maize-rice cropping system. This rate of P also significantly reduced P accumulation in the soil, thereby reducing the chances of negative effects on soil nutrient balance/availability. The results suggested the need for a cropping systems approach to conserve and effectively use native and fertilizer P in the sweet pepper-rice cropping system.