S. De Neve

Influence of soil compaction on carbon and nitrogen mineralization of soil organic matter and crop residues.

Abstract We studied the influence of soil compaction in a loamy sand soil on C and N mineralization and nitrification of soil organic matter and added crop residues. Samples of unamended soil, and soil amended with leek residues, at six bulk densities ranging from 1.2 to 1.6 Mg m–3 and 75% field capacity, were incubated. In the unamended soil, bulk density within the range studied did not influence any measure of microbial activity significantly. A small (but insignificant) decrease in nitrification rate at the highest bulk density was the only evidence for possible effects of compaction on microbial activity. In the amended soil the amounts of mineralized N at the end of the incubation were equal at all bulk densities, but first-order N mineralization rates tended to increase with increasing compaction, although the increase was not significant. Nitrification in the amended soils was more affected by compaction, and NO3–-N contents after 3 weeks of incubation at bulk densities of 1.5 and 1.6 Mg m–3 were significantly lower (by about 8% and 16% of total added N, respectively), than those of the less compacted treatments. The C mineralization rate was strongly depressed at a bulk density of 1.6 Mg m–3, compared with the other treatments. The depression of C mineralization in compacted soils can lead to higher organic matter accumulation. Since N mineralization was not affected by compaction (within the range used here) the accumulated organic matter would have had higher C : N ratios than in the uncompacted soils, and hence would have been of a lower quality. In general, increasing soil compaction in this soil, starting at a bulk density of 1.5 Mg m–3, will affect some microbially driven processes.

Modelling N mineralization of vegetable crop residues during laboratory incubations

Abstract A range of vegetable crop residues were subjected to a modified Stevenson chemical fractionation. N mineralization parameters were determined for the fractionated crop residue components from aerobic laboratory incubations. Fresh chopped crop residues were incubated with soil at fixed temperature and soil moisture content. N mineralization (both NH4+N and NO3−N) was measured over a 3–4 month period (depending on degradability) at regular intervals. The first order kinetic model N(t) = NA (1 − e−kt) was fitted to the mineralization data of total N (parameters NA and k) and of organic N (parameters NA,org and korg). Both the amount of mineralizable N (NA) and the rate constant k differed widely between the residues. The amount of mineralizable organic N (NA,org) was better correlated to chemical composition than the amount of mineralizable total N (NA). The parameter NA,org was best related to the C-to-N ratio of the lignin (78% of the total variance of NA,org explained). Good correlations were also observed with lignin content (r = 0.863) and with the water soluble fraction (r= 0.861). The rate constant korg was much less predictable. It was related negatively to the relative amount of organic N (relative to total N) contained in the residues. The model presented can be used to calculate the amount of N mineralized at any specified time after incorporation of the residues under the experimental conditions used. Prediction of N mineralization improves with time of incubation of the residues because the influence of the rate constant diminishes. The critical C-to-N ratio, i.e. the break point between net N mineralization and net N immobilization (NA = 0), was calculated to be at a C-to-N ratio of 44.

Temperature effects on C- and N-mineralization from vegetable crop residues

Net N-mineralization and nitrification from soil organic matter and from vegetable crop residues (leaf-blades of cauliflower and stems of red cabbage) were measured at 4 temperatures during aerobic incubation in the laboratory. C-mineralization from leaf-blades of cauliflower was monitored at 3 different temperatures. N-mineralization from soil organic matter was best described by zero order kinetics N(t)=kt whereas N- and C-mineralization from the crop residues were described by single first order kinetics. Stems of red cabbage mineralized much more slowly than leaf-blades of cauliflower. S-shaped functions were fitted to the relationship between the rate constants of both C and N-mineralization and temperature. The rate parameter κ of the S-shaped function reflects the temperature dependence of the mineralization rate k. The parameter κ for N-mineralization of the stem material (κ=5.36) was significantly higher than for the leaf-blades (κ=3.38), indicating that there is a strong interaction between temperature and resistance to degradation in the soil. N-mineralization from soil organic matter was least sensitive to temperature (κ=2.63). Temperature dependence of nitrification was not significantly different from mineralization over the temperature range considered. Rate constants for C-mineralization of cauliflower leaf-blades were higher than for N-mineralization, but the temperature dependence of the rate constants was not significantly different for both processes.

Measured and Simulated Nitrate Leaching on an Organic and a Conventional Mixed Farm

ABSTRACT A number of fields on an organic (a potato field, a pasture and a field with red cabbage) and a conventional mixed farm (a potato field, a pasture and a field with cauliflower) were compared with respect to nitrate leaching in winter, using both soil mineral N measurements and a simulation approach. Soil organic matter N mineralization rates of the surface layers were determined during laboratory incubations. Mineral N contents of the 0–90 cm layer of each field were monitored during the period October-March. A coupled N mineralization—leaching model was used to simulate NO3-leaching over this period. Calculated NO3-leaching was in the same range on most fields, between 66 and 87 kg NO3-N ha−1, except for the conventional pasture, which had smaller losses (35 kg NO3-N ha−1), and the conventional cauliflower, which had very large losses (293 kg NO3-N ha−1). The contribution of N mineralization during autumn and winter to NO3 leaching was important, and could only be taken into account explicitly using simulation models. The most remarkable difference with respect to N leaching was observed between the conventional cauliflower and organic red cabbage field, both with very high N fertilization rates. This was attributed to the differences in type of fertilizer applied, with N fertilization on the conventional fields much more conducive to N losses. The average NH4 +-N concentrations during the field experiment were larger on the organic fields than on the conventional fields. This indication for a retardation or inhibition of nitrification in the organic fields is an important observation, which obviously could have important implications for NO3 − leaching, but needs to be further investigated and quantified before it could be incorporated into simulation models.

Fractionation of vegetable crop residues in relation to in situ N mineralization

The mineralization of N from soil organic matter and from incorporated crop residues is the most uncertain factor in modern N recommendation systems. In the vegetable growing region of West Flanders large amounts of crop residues are ploughed in yearly. In a field trial, crop residues of six vegetable crops were incorporated into a soil and the evolution of mineral N was followed to a depth of 120 cm. Stevenson fractionation was applied to all crop residues. An incubation experiment was set up using five of the crop residues of the field trial. Results of the incubation experiment and the field trial were in good agreement. Linear regressions were calculated between amount of N mineralized in the field and chemical properties. Highly significant correlations (p < 0.001) were obtained between net amount of N mineralized after two weeks and lignin content (R2 = 0.949). The net amount of N mineralized after four weeks was best correlated with the C:N ratio times the lignin content over the square root of the water soluble carbohydrates (r2 = 0.862, p = 0.001).

Opportunities and barriers to on-farm composting and compost application: A case study from northwestern Europe.

Maintaining and increasing soil quality and fertility in a sustainable way is an important challenge for modern agriculture. The burgeoning bioeconomy is likely to put further pressure on soil resources unless they are managed carefully. Compost has the potential to be an effective soil improver because of its multiple beneficial effects on soil quality. Additionally, it fits within the bioeconomy vision because it can valorize biomass from prior biomass processing or valorize biomass unsuitable for other processes. However, compost is rarely used in intensive agriculture, especially in regions with high manure surpluses. The aim of this research is to identify the barriers to on-farm composting and the application of compost in agriculture, using a mixed method approach for the case of Flanders. The significance of the 28 identified barriers is analyzed and they are categorized as market and financial, policy and institutional, scientific and technological and informational and behavioral barriers. More specifically, the shortage of woody biomass, strict regulation, considerable financial and time investment, and lack of experience and knowledge are hindering on-farm composting. The complex regulation, manure surplus, variable availability and transport of compost, and variable compost quality and composition are barriers to apply compost. In conclusion, five recommendations are suggested that could alleviate certain hindering factors and thus increase attractiveness of compost use in agriculture.

Neem seed oil: a potent nitrification inhibitor to control nitrate leaching after incorporation of crop residues

The effect of neem seed oil and neem leaf extract as organic nitrification inhibitors (NIs) on the accumulation of and , and nitrification inhibition after incorporation of crop residue was investigated in an incubation experiment. Dicyandiamide (DCD) applications of 15 and 30 kg active ingredient ha− 1 were used as low and high doses of a synthetic NI. Soil samples were amended with 21 g kg− 1 cauliflower leaves and treated with NIs at a rate of 30 kg ha− 1 of neem seed oil, 60 kg ha− 1 of neem leaf extract, 15 kg ha− 1 of DCD, and 30 kg ha− 1 DCD. Samples were incubated at temperatures corresponding to field temperatures during fall and winter in Flanders, Belgium. Neem seed oil increased accumulation by 8.9 mg kg− 1 and decreased by 13.5 mg kg− 1 within a month. High and low doses of DCD increased accumulation by 48.2 and 1.6 mg kg− 1, respectively. Nitrification was inhibited by 29% for 30 days by the low dose of DCD, 58% in 30 days by neem seed oil, and 42% in 45 days by the high dose of DCD. Nitrification was not inhibited by the neem leaf extract.

Potential of chopped heath biomass and spent growth media to replace wood chips as bulking agent for composting high N-containing residues

We investigated the potential of C-rich byproducts to replace wood chips as bulking agent (BA) during composting. The impact of these alternatives on the composting process and on compost stability and characteristics was assessed. Three BA (chopped heath biomass and spent growth media used in strawberry and tomato cultivation) were used for processing leek residues in windrow composting. All BA resulted in stable composts with an organic matter (OM) content suitable for use as soil amendment. Using chopped heath biomass led to high pile temperatures and OM degradation and a nutrient-poor compost with high C/P ratio appropriate for increasing soil organic carbon content in P-rich soils. Spent substrates can replace wood chips, however, due to their dense structure and lower biodegradation potential, adding a more coarse BA is required. Generally, the nutrient content of the composts with growth media was higher than the composts with wood chips and chopped heath biomass.

Field storage conditions for cattle manure to limit nitrogen losses and optimise fertiliser value

Storage and application of cattle farmyard manure (CFM) can cause considerable environmental problems through nutrient losses to soil, water and air, if not properly handled. We investigated different storage conditions of CFM at field scale to reduce nitrogen (N) losses to the soil, meanwhile optimising the agronomical quality of the CFM. The treatments differed in terms of storage method (stockpiling, extensive composting or co-composting with bulking agents) and coverage (no cover, plastic or geotextile cover). Over the different treatments, the ammonium-N concentrations under the piles in the 0–90 cm soil layer amounted to a maximum of 4.2% of the initial manure N content. We were able to assess the relative importance of each of the two processes resulting in a higher mineral N concentration under the piles, i.e. direct leaching from the CFM to the soil on the one hand, and a smaller indirect effect of elevated soil temperatures (up to 37°C) under the piles resulting in higher N mineralisation in the top soil on the other hand. NH4+-N was the most important component of mineral N under all heaps due to limited oxygen diffusion to the soil. N leaching and end-product quality were affected by a combination of treatment option (i.e. storage and cover) and initial manure characteristics. When CFM was characterised by a low volumetric moisture content and high C : N ratio, so in case of straw-rich CFM or CFM with added bulking agents, composting led to the least N leaching and most stable end product. When CFM was characterised by a high volumetric moisture content and low C : N ratio, stockpiling and covering (plastic or geotextile) resulted in lower N leaching to the soil. Stockpiling and covering the CFM with a geotextile resulted in a more stable end product than did covering with a plastic.