Georges Hofman

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.

Mitigation options to reduce phosphorus losses from the agricultural sector and improve surface water quality: a review

The EU Water Framework Directive (WFD) obliges Member States to improve the quality of surface water and groundwater. The measures implemented to date have reduced the contribution of point sources of pollution, and hence diffuse pollution from agriculture has become more important. In many catchments the water quality remains poor. COST Action 869 was an EU initiative to improve surface water quality that ran from 2006 to 2011, in which 30 countries participated. Its main aim was a scientific evaluation of the suitability and cost-effectiveness of options for reducing nutrient loss from rural areas to surface waters at catchment scale, including the feasibility of the options under different climatic and geographical conditions. This paper gives an overview of various categories of mitigation options in relation to phosphorus (P). The individual measures are described in terms of their mode of action, applicability, effectiveness, time frame, environmental side-effects (N cycling) and cost. In total, 83 measures were evaluated in COST Action 869.

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.

Progress in Nitrogen Cycling Studies

N research effort has undergone major changes over recent decades with changing emphasis because of environmental problems and issues. This driving force, coupled with a universal desire to improve N-use efficiency, appreciation of the importance of maintaining soil resource quality and a need to provide integrated landscape managements, will continue to prompt new research areas and issues for study. Already, much information has been provided and new approaches and needs defined. It will be essential in future research to take full note of the many interactions that occur and to provide a mechanistic basis so that scaling of effects can be undertaken with the appropriate simplification without being superficial. Examples of interactions, as well as fundamental gaps in the basic processes are discussed and needs for future research identified.

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.

Phosphate enrichment in the sandy loam soils of West-Flanders, Belgium

The last three decades, pig breeding has evolved towards a specialised, large scaled, land independent bio-industry in the province of West-Flanders. Subsequently, in certain regions, very high amounts of liquid pig manure are produced each year. This pig slurry is used as a fertilizer at a rate which very often exceeds normal agricultural practices. Because of the nonequilibrium between the phosphorus crop requirements and the P-inputs, phosphates accumulate in the soil. However, the phosphate sorption capacity of a soil is limited. Once the sorption capacity is exceeded, phosphates will start leaching through the soil profile. Since, during winter, in these areas, the groundwater table is situated at a depth of less than 1.0 m, phosphate breakthrough might take place. In the sandy loam soil region (± 1000 km2) of the province, an inventory of the P status of the soil was made. The region was sampled according to a regular grid with 2 km intervals. At random, some sample points were only 500 m apart. This resulted in a total of 296 samplings. In view of fertilizer recommendations, lactate extractable P of the plough layer (0–30 cm) was determined. A maximum value of 101 mg P 100 g-1 of air dry soil, a minimum value of 6 mg P 100 g-1 and a median value of 31 mg P 100 g-1 were found, indicating that for half of the spots monitored, the P status of the soil is high to very high. An oxalate extraction was done to investigate the phosphate saturation of the soil profile (0–90 cm). Based on a critical phosphate saturation degree of 30%, more than half of the soil profiles are phosphate saturated. Phosphate leaching at a rate higher than 0.1 mg ortho-P l-1 at a depth of 90 cm can be expected. Therefore, a restriction of the P fertilization should be highly recommended. The geostatistical processing of the data using block kriging resulted in a spatial continuous estimate of the phosphate saturation degree. A good agreement was found between the pig density and the phosphate saturation degree of the soil profile.

Carbon mineralization from composts and food industry wastes added to soil

We have studied the short term C mineralization of six wastes from important food industries, one sludge from a biogas plant and three composts. All the wastes were characterized chemically and fractionated according to the Van Soest method. The fresh wastes were incubated under controlled environment conditions to determine the C mineralization rate. Based on first order mineralization kinetics, we calculated the hypothetical amount of stable C in the wastes as the amount of C that would not be mineralized within one year under field conditions. The percentage of stable organic C in the organic matter was in general much larger in the composts than in the other wastes, but when expressed on dry matter, the non-composted wastes had comparable or larger amounts of stable organic C than the composts and have a considerable potential for supplying organic matter to soils, and hence for C sequestration. The amount of stable organic C could best be predicted by the total N content of the wastes (Ra2 = 0.855), whereas the results of the fractionation had very little predictive power, probably due to problems related to the high ash content of some of the wastes. An index that combined stable organic C and N and P content in the wastes was calculated to assess possible limitations for applying these wastes in agriculture. Under current nutrient legislation in Western Europe, a number of these wastes will only be usable in small amounts, but these and other food industry wastes could still prove to be valuable soil amendments in nutrient poor situations, for increasing soil organic C content and supplying nutrients.

Simulation of net N immobilisation and mineralisation in substrate-amended soils by the NCSOIL computer model

Abstract The capability of the NCSOIL computer model to simulate the effects of residue fractions on mineralisation-immobilisation turnover was evaluated. Heterogeneous organic substrates were represented in the model by three Van Soest pools, decomposing at different rates. Dried and ground wheat straw, sunflower stalks, wheat stubble and sheep manure (5.22 g kg–1 soil) were respectively added to a Chromic Calcixerert and aerobically incubated for 224 days at 22±2  °C and 75% field capacity. The CO2 evolution rates peaked shortly after the C amendments were added, with the highest rate in the sunflower- stalk-amended soils. The addition of organic substrates induced rapid N immobilisation. Net mineralisation was detected earliest in the sunflower-stalk treatment (day 14), while soils with the other amendments showed no net N mineralisation until day 52. The NCSOIL model was calibrated for this soil with CO2 and inorganic N data from the control soil, yielding a χ2 value of 0.011. The overestimation by the model of the C mineralisation data in the case of C-amended soils clearly showed that the concept of three Van Soest pools, decomposing independently at a specific rate constant, is not valid. A retardation factor, that was related to the lignin content of the decomposing material, was introduced into the model. After its introduction the model satisfactorily simulated the C mineralisation rates. However, for all plant residues, N mineralisation was underestimated towards the end of the incubation period. In the case of the soil amended with sheep manure, there was a large discrepancy between simulated and experimental N mineralisation-immobilisation kinetics, suggesting a different allocation of N in animal manure to N-containing fractions compared to that of plant residues. The results indicated that a N fractionation procedure for organic residues should be tested and incorporated into the model.

Quantification of organic carbon in soils: A comparison of methodologies and assessment of the carbon content of organic matter

Abstract Renewed interest in temporal soil organic carbon (SOC) stock changes has stressed the importance of reliable methods for quantitative assessment of organic compound (OC) content. Particularly with the establishment of modern dry‐combustion analyzers, which are replacing the traditional wet‐oxidation methods, the need for correct relationships between both is of crucial importance for comparison of past and current SOC data in long‐term SOC stock change studies. Dry combustion with a Variomax CNS‐analyzer was the standard to evaluate three other methods for Belgian agricultural soils. Excellent linear relationships were found with the Walkey and Black method and the Springer and Klee method, whereas a Shimadzu TOC‐analyzer slightly underestimated the OC content. Precision of the investigated methods was comparable and tended to be dependent on the sample size used for measurement. The OC oxidation efficiency of the most widely applied method of Walkey and Black for the soils in this study was very close to the generally accepted 75%. Mass loss on ignition at 800°C could be very well related to the soil OC content and the clay content. The traditional factor of 1.724 used to convert OC measurements to organic matter percentages is not valid for the investigated soils, which demonstrates that rather regional‐specific factors (in this study 1.911) should be determined and adopted.

A methodology for the calculation of farm level nitrogen and phosphorus balances in Flemish agriculture

Abstract Since the early 1990s, Flemish agriculture has been confronted with serious nutrient problems. Recently, the Flemish Government started encouraging farmers to take responsibility for the manure problem. This was done by giving them the choice between a manure administration system, based on fixed standards, or a nutrient balance system. In this project a methodology for the composition of nutrient balances at farm level in Flanders was developed. This methodology defines input–output models for the different sectors (cattle, pigs and poultry) and fixes rules on how to determine quantities and nutrient contents for several products. In addition, nutrient flows were followed during 1 year on 40 Flemish farms in different sectors and a mineral balance was calculated for each of them. Four major restrictions to the accurate calculation of farm level nutrient balances were identified: (1) the wide variability that is allowed between actual and reported nutrient composition of concentrated feed; (2) the estimates of the amount and composition of manure; (3) the assessment of changes in standing stock on the farm between the beginning and end of the reporting period and (4) the accuracy of the data supplied by the farmers.