Steven Sleutel

Biochar amendment to soils with contrasting organic matter level: effects on N mineralization and biological soil properties

Four biochar types, produced by slow pyrolysis of poultry litter (PL) and pine chips (P) at 400 or 500 °C, were added to two adjacent soils with contrasting soil organic matter (SOM) content (8.9 vs. 16.1 g C kg−1). The N mineralization rate was determined during 14‐week incubations and assessments were made of the microbial biomass C, dehydrogenase activity, and the microbial community structure (PLFA‐extraction). The addition of PL biochars increased the net N mineralization (i.e., compared to the control treatment) in both soils, while for treatments with P biochars net N immobilization was observed in both soils. Increasing the pyrolysis temperature of both feedstock types led to a decrease in net N mineralization. The ratio of Bacterial to Fungal PLFA biomarkers also increased with addition of biochars, and particularly in the case of the 500 °C biochars. Next to feedstock type and pyrolysis temperature, SOM content clearly affected the assessed soil biological parameters, viz. net N mineralization or immobilization, MBC and dehydrogenase activity were all greater in the H soil. This might be explained by an increased chance of physical contact between the microbial community activated by SOM mineralization upon incubation and discrete biochar particles. However, when considering the H soils double C and N content, these responses were disproportionally small, which may be partly due to the L soils, somewhat more labile SOM. Nonetheless, increasing SOM content and microbial biomass and activity generally appears to result in greater mineralization of biochar. Additionally, higher N mineralization after PL addition to the H soil with lower pH than the L soil can be due to the liming effect of the PL biochars.

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.

Modeling Soil Moisture Effects on Net Nitrogen Mineralization in Loamy Wetland Soils

Nutrient dynamics in wetland ecosystems are largely controlled by soil moisture content. Therefore, the influence of soil moisture content on N mineralization should be explicitly taken into account in hydro-ecological models. The aim of this research was to establish relationships between N mineralization and soil moisture content in loamy to silty textured soils of floodplain wetlands in central Belgium. Large undisturbed soil cores were taken, incubated for 3 months under various moisture contents, and zero order and first order N mineralization rates were calculated. We used the percentage water-filled pore space (WFPS) as an expression of soil moisture because it is a better index for aeration dependent biological processes than volumetric moisture content or water retention. The relationship between the N mineralization rate and %WFPS was described by a Gaussian model. The optimum WFPS for N mineralization ranged between 57% and 78%, with a mean of 65% ± 6% WFPS. Expected annual net N mineralization rates at field temperature (9.7°C) and at optimal moisture content varied between 30 and 186 kg N ha-1 (0–15 cm depth) year-1, with a mean of 110 ± 42 kg N ha-1 (0–15 cm) year-1. The mean N turnover rate amounted to 2.3 ± 1.1 g N 100 g-1 N year-1. Multiple linear regressions between N mineralization and general soil parameters showed that soil structure has an overriding impact on N mineralization in wetland ecosystems.

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.

Carbon mineralisation and pore size classes in undisturbed soil cores

Soil pore network effects on organic matter turnover have, until now, been studied indirectly because of lack of data on the 3D structure of the pore network. Application of X-ray computed tomography (X-ray CT) to quantify the distribution of pore neck size and related pore sizes from undisturbed soil cores, with simultaneous assessment of carbon (C) mineralisation, could establish a relationship between soil organic matter (SOM) decomposition and soil pore volumes. Eighteen miniature soil cores (diameter 1.2 cm, height 1.2 cm) covering a range of bulk densities were incubated at 20°C for 35 days. Respiration was modelled with a parallel first- and zero-order kinetic model. The cores were scanned at 9.44 µm resolution using an X-ray CT scanner developed in-house. Correlation analysis between the slow pool C mineralisation rate, ks, and pore volume per pore neck class yielded significant (P  350 µm pore neck classes, respectively. Because larger pores are most probably mainly air-filled, a positive relation with ks was ascribed to enhanced aeration of smaller pores surrounding large pores. The weak and insignificant relationship between the smallest pore neck class (<9.44 µm) and ks could be explained by obstructed microbial activity and mobility or diffusion of exo-enzymes and hydrolysis products as a result of limited oxygen availability. This study supports the hypothesis that the impact of soil structure on microbial processes occurs primarily via its determination of soil water distribution, which is possibly the main driver for the location of C mineralisation in the soil matrix.

Increasing trends of dissolved organic nitrogen (DON) in temperate forests under recovery from acidification in Flanders, Belgium

We evaluated trends (2005-2013) and patterns of dissolved organic nitrogen (DON) and its ratio with dissolved organic carbon (DOC), DOC:DON in atmospheric deposition and soil solution of five Level II plots of the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) in Flanders, Northern Belgium. The primary aim was to confirm positive postulated trends in DON levels and DOC:DON under on-going recovery from acidification. The DON concentrations (0.95-1.41 mg L(-1)) and fluxes (5.6-8.3 kg ha(-1)y(-1)) in throughfall were about twice as high compared to precipitation in the open field (0.40-0.48 mg L(-1), 3.0-3.9 kg ha(-1)y(-1)). Annual soil profile leaching losses of DON varied between 1.2 and 3.7 kg ha(-1)y(-1). The highest soil DON concentrations and fluxes were observed beneath the O horizon (1.84-2.36 mg L(-1), 10.1-12.3 kg ha(-1)y(-1)). Soil solution concentrations and fluxes of DON showed significant increasing trends. Temporarily soil solution DOC:DON rose following an exceptionally long spring drought in 2007, suggesting an effect of drying and rewetting on DOM composition. Further research is needed to test the dependence of DON and DOC:DON on factors such as latitude, forest cover, length of the growing season, hydrology and topography. Nonetheless, even with considerable variation in soil type, level of base saturation, and soil texture in the five included ICP Forests Level II plots, all data revealed a proportionally larger positive response of DON flux than DOC to recovery from acidification.

Limited influence of tillage management on organic matter fractions in the surface layer of silt soils under cereal–root crop rotations

Reduced tillage (RT) management may increase surface soil organic carbon (SOC) and nitrogen (N), particularly due to accumulation of labile organic matter (OM). We investigated the effect of RT compared with conventional tillage (CT) on the distribution of SOC and N over different soil fractions from 7 pairs of fields with cereal–root crop rotations, in the Belgian loess belt. Surface soil samples (0–100 mm) were physically fractionated according to a sequential sieving and density separation method into stable microaggregates, silt and clay, and free and occluded particulate OM fractions. RT management was previously found effective in increasing the organic C and organic N content of the surface soil (0–100 mm) at these 7 sites. Here, physical fractionation showed that the difference in amount of organic C and N in free particulate OM (fPOM), intra-microaggregate particulate OM (iPOM), and silt and clay associated OM between the RT and CT soils contributed 34, 29, and 37% of the increase in SOC and 35, 32, and 33% of the increase in N. The contribution of OC and N in iPOM and fPOM increased significantly on a relative basis under RT management. Only a modest increase in iPOM and slight enhancement of microaggregation was observed in RT compared with CT soils. We suggest that the repeated disturbance of soil by harvest of root crops and repeated use of cultivators and harrows may limit the accumulation of physically protected POM under RT management of these Western European cereal–root crop rotations. Instead, most of the accumulated OC and N in the surface horizons under RT management is present as free unprotected POM, which could be prone to rapid loss after (temporary) abandonment of RT management.

Soil Nutrient Status of Organic Farms in Flanders: An Overview and a Comparison with the Conventional Situation

ABSTRACT Organic farming is put forward as a sustainable alternative to the conventional way of farming. However, until now there has been practically no research on soil related aspects of organic farming in Flanders. Therefore, in this study, the carbon, nitrogen, sulphur and phosphorus status of the fields on organic farms was measured and was related to characteristics of the farms involved (land use, soil texture, management intensity and time since conversion to organic farming) and was, where possible, compared with the general soil fertility status of agricultural soils in Flanders. The SOC (soil organic carbon) contents tended to be larger in the fields of the organic farms studied than in conventional farming. This content was correlated positively with the management intensity of the fields. However, some of the organic fields still had rather low SOC contents. To increase this content organic farmers will need to apply more organic materials with large C:N and C:P ratios taking into account the strict current manure legislation. The mineral N contents on the organic farms measured in the period between 1 October 2003 and 15 November 2003, indicating the risk of nitrate leaching, were well below the average values of the different land use classes of conventional agricultural soils in East Flanders. The available P content of the organic fields was high to very high as is also the case for conventional agriculture in East Flanders and was strongly related to the management intensity.

Quantifying the Contribution of Entire Free-Living Nematode Communities to Carbon Mineralization under Contrasting C and N Availability

To understand the roles of nematodes in organic matter (OM) decomposition, experimental setups should include the entire nematode community, the native soil microflora, and their food sources. Yet, published studies are often based on either simplified experimental setups, using only a few selected species of nematode and their respective prey, despite the multitude of species present in natural soil, or on indirect estimation of the mineralization process using O2 consumption and the fresh weight of nematodes. We set up a six-month incubation experiment to quantify the contribution of the entire free living nematode community to carbon (C) mineralization under realistic conditions. The following treatments were compared with and without grass-clover amendment: defaunated soil reinoculated with the entire free living nematode communities (+Nem) and defaunated soil that was not reinoculated (-Nem). We also included untreated fresh soil as a control (CTR). Nematode abundances and diversity in +Nem was comparable to the CTR showing the success of the reinoculation. No significant differences in C mineralization were found between +Nem and -Nem treatments of the amended and unamended samples at the end of incubation. Other related parameters such as microbial biomass C and enzymatic activities did not show significant differences between +Nem and -Nem treatments in both amended and unamended samples. These findings show that the collective contribution of the entire nematode community to C mineralization is small. Previous reports in literature based on simplified experimental setups and indirect estimations are contrasting with the findings of the current study and further investigations are needed to elucidate the extent and the mechanisms of nematode involvement in C mineralization.

Effect of cultivation systems on the distribution of soil organic matter in different fractions

The most important function of agriculture is to supply food for the population. This role is permanently endangered by the degradation of soil structure and the loss of organic matter. This is the reason why it is substantial to preserve the soil organic matter, because carbon is the most important element of food chain. Some tillage operations may decrease the total soil organic matter (SOM), but detailed changes for different SOM fractions are less well known. Individual SOM fractions may be more sensitive indicators to the changes in management than total organic C (OC) and N. SOM fractions can be isolated and measured by physical fractionation of soil (Cambardella et Elliott (1992), Janzen et al. (1992)). SOM can be protected in aggregates, but conventional or intensive tillage can cause the disruption of these aggregates (Cambardella et Elliott (1994)) which causes loss of OM. Six et al (2000a) found