Organic Matter in Clay Density Fractions from Sandy Cropland Soils with Differing Land-Use History

Land-use history is oft en overlooked when assessing soil fertility of intensive cropland production systems. Th e unusually high organic carbon (OC) content of many sandy cropland soils in Northwestern Europe is unexpected given their general low clay content (3–8%) and organic matter (OM) input typical of cropland, but appears to be related to historical heathland land-use. Clay fraction OM composition was compared between two groups of sandy cropland soils with (HC) or without (CC) a history of heathland/forest land-use. Light (1.6–2.2 g cm−3) and heavy (>2.2 g cm−3) clay fractions in HC soils were nearly twice as rich in OC (on average 199 g kg−1) compared with those of CC soils (on average 109 g kg−1). Th e hypothesized preferential presence of stable heathland derived OM in light soil fractions, was not supported by our data. Pyrolysis-fi eld ionization mass spectrometry of the clay fractions revealed a more decomposed character of OM in the CC soils and lasting long-term infl uence of land-use history on SOM composition. Th is could be concluded from higher proportions of lipids and sterols, a lower thermostability in the HC compared with the CC soils, and enrichment of alkylaromatics and heterocyclic N-containing compounds in the latter. Th e density fractionation methodology separated organic-mineral particles with similar OM loadings but lower proportions of sterols and medium to long-chained lipids in the heavy compared with the light clay fraction. Given the very high clay OC loadings (6–16 mg C m−2) and low binding capacity of the quartz/kaolinite/mica dominated clays, we hypothesize that OM–OM interactions are involved as an OM stabilization mechanism. However, contrary to our hypothesis high clay OC loading (and hence thick OM layering) were found in all sandy croplands regardless of land-use history or density fraction

Organic matter fractions and N mineralization in vegetable-cropped sandy soils

Soil organic nitrogen mineralization rates and possible predictors thereof were investigated for vegetable-growing soils in Belgium. Soil organic matter (SOM) was fractionated into sand (> 53 lm) and silt+clay (< 53 lm) fractions. The latter fraction was further separated into 6%NaOCl-oxidation labile (6%NaOCl-ox) and resistant N and C and subsequently into 10%HF-extractable (mineral bound) and resistant (recalcitrant) N and C. The N mineralization turnover rate (% of soil N/year) correlated with several of the investigated N or C fractions and stepwise linear regression confirmed that the 6%NaOCl-ox N was the best predictor. However, the small R² (0.42) of the regression model suggests that soil parameters other than the soil fractions isolated here would be required to explain the significant residual variation in N mineralization rate. A next step could be to look for alternative SOM fractionations capable of isolating bioavailable N. However, it would appear that the observed relationships between N fractions and N mineralization may not be causal but indirect. The number of vegetable crops per rotation did not influence N mineralization, but it did influence 6%NaOCl-ox N, probably as an effect of differences in crop residues returned and organic manure supply. However, the nature of this relation between management, SOM quality and N mineralization is not clear. Explanation of correlations between N mineralization and presumed bioavailable N fractions, like the 6%NaOCl-ox N, requires further mechanistic elucidation of the N mineralization process

Soil organic matter fractionation as a tool for predicting nitrogen mineralization in silty arable soils

After decades of searching for a practical method to estimate the N mineralization capacity of soil, there is still no consistent methodology. Indeed it is important to have practical methods to estimate soil nitrogen release for plant uptake and that should be appropriate, less time consuming, and cost effective for farmers. We fractionated soil organic matter (SOM) to assess different fractions of SOM as predictors for net N mineralization measured from repacked (disturbed) and intact (undisturbed) soil cores in 14 weeks of laboratory incubations. A soil set consisting of surface soil from 18 cereal and root-cropped arable fields was physically fractionated into coarse and fine free particulate OM (coarse fPOM and fine fPOM), intra-microaggregate particulate OM (iPOM) and silt and clay sized OM. The silt and clay sized OM was further chemically fractionated by oxidation with 6% NaOCl to isolate an oxidation-resistant OM fraction, followed by extraction of mineral bound OM with 10% HF (HF-res OM). Stepwise multiple linear regression yielded a significant relationship between the annual N mineralization (kg N⁄ha) from undisturbed soil and coarse fPOM N (kg N⁄ha), silt and clay N (kg N⁄ha) and its C:N ratio (R2 = 0.80; P < 0.01). The relative annual N mineralization (% of soil N) from disturbed soils was related to coarse fPOM N, HF-res OC (% of soil organic carbon) and its C:N ratio (R2 = 0.83; P < 0.01). Physical fractions of SOM were thus found to be the most useful predictors for estimating the annual N mineralization rate of undisturbed soils. However, the bioavailability of physical fractions was changed due to the disturbance of soil. For disturbed soils, a presumed stable chemical SOM fraction was found to be a relevant predictor indicating that this fraction still contains bio-available N. The latter prompted a revision in our reasoning behind selective oxidation and extraction as tools for characterizing soil organic N quality with respect to N availability. Nonetheless, the present study also underscores the potential of a combined physical and chemical fractionation procedure for isolating and quantifying N fractions which preferentially contribute to bulk soil N mineralization. The N content or C:N ratio of such fractions may be used to predict N mineralization in arable soils