Protection Of Soil Organic Matter Research Articles

Importance of macroaggregate dynamics in controlling soil carbon stabilization: short-term effects of physical disturbance induced by dry–wet cycles

Physical protection of soil organic matter by aggregates is considered to be an important mechanism for soil carbon stabilization. In this study, we evaluated the effect of drying and wetting on the interrelationships between macroaggregate formation and degradation (i.e. macroaggregate turnover), microaggregate formation within macroaggregates, and aggregate-associated carbon dynamics. Dry–wet (DW) cycles were used to simulate one of the soil aggregate disruptive effects induced by tillage. A conceptual model developed from long-term no-till (NT) and conventional tilled (CT) field experiments was then used to interpret our results. Sieved (250μm) air-dried soil samples were taken from Weld silt loam soil (Aridic Paleustoll) that had been cultivated continuously. The samples were mixed with 13C-labeled wheat and incubated for 74days. One set of soil samples was subjected to four DW cycles, while the other set was kept at field capacity (control). At days 14, 44 and 74, water-stable microaggregates (53–250μm) held within large macroaggregates (>2000μm) were isolated. Inter-and intra-microaggregate particulate organic matter (POM) fractions were separated and analyzed for total and wheat-derived C. After two DW cycles (day 44), we observed a significantly lower proportion of water-stable microaggregates within DW macroaggregates compared to control macroaggregates (9 versus 13% of the macroaggregate weight). Simultaneously, DW macroaggregates had significantly lower intra-microaggregate POM-C concentrations compared to control macroaggregates. This difference in intra-microaggregate POM-C between DW and control was more significant for native intra-microaggregate POM-C (0.73 versus 1.03gkg−1 macroaggregates) (P<0.05) than for wheat-derived intra-microaggregate POM-C (41 versus 49mg C kg−1 macroaggregates) (P<0.1). After two DW cycles (day 44), drying and wetting no longer caused macroaggregate disruption. From day 44 to day 74, both the proportion of microaggregates and the concentration of intra-microaggregate POM-C significantly increased in DW macroaggregates. We conclude that POM-C in new microaggregates within macroaggregates is inhibited by an enhanced macroaggregate turnover, which is only in the short term enhanced by drying and wetting. Furthermore, we suggest that besides a release of total (i.e. native and wheat-derived) POM upon macroaggregate breakdown, drying and wetting induced a fast reformation of macroaggregates with preferential incorporation of wheat-derived POM, resulting in a relative decline of native POM-C in DW macroaggregates.

Carbon and Nitrogen in the Enriched Labile Fraction along a Climosequence of Zonal Steppe Soils in Russia

Breaking aggregates by plowing resulted in the decomposition of formerly physically protected soil organic matter (SOM), including the enriched labile fraction (ELF), but it was unknown to what extent such effects were controlled by climate. To investigate this question, aggregate‐size fractions were obtained from each of five native and adjacent long‐term cultivated topsoils across a climosequence in the Russian steppe. After ultrasonic dispersion of small macro‐ (250–2000 μm) and microaggregates (53–250 μm) at 100 and 500 J mL−1 (only for so‐called stable macroaggregates) and a particle‐size fractionation, density fractions &lt;1.85 g cm−3, 1.85 to 2.07 g cm−3, 2.07 to 2.22 g cm−3 (= ELF for the small macroaggregates) and &gt;2.22 g cm−3 were obtained from the fine silt–sized particles. In all fractions, C and N contents were determined. The stable aggregates were found only at native sites, were almost free of ELF, and showed their C maximum in the two lightest fractions. Cultivation reduced the C and N contents in all aggregates. In small macroaggregates, C losses occurred primarily as ELF, whereas microaggregates lost C in the fine silt density range of 1.85 to 2.22 g cm−3 The C partition among the fine silt density fractions was not related to climate. Losses in ELF were also not related to climate, suggesting that ELF represents a C pool that is site‐specifically influenced by cultivation. The C losses from fractions &lt;2.07 g cm−3, however, increased as the climate became dryer and warmer, suggesting that they reveal interactive effects of climate and land use on physically stabilized SOM.

Influences of mycelial fungi on soil aggregation and organic matter storage in conventional and no-tillage soils

Soil microbial community composition, aggregation and organic matter (SOM) content can be markedly influenced by tillage and crop management practices. This study was undertaken to determine to what extent differences in populations of mycelial fungi in conventional (CT) and no-tillage (NT) soils contribute to soil aggregation and soil organic matter storage. Fungicide (Captan) and control treatments were established in long-term CT and NT plots on a well-drained Hiwassee sandy clay loam soil (clayey kaolinitc thermic Rhodic Kanhapludult). The effects of these treatments on total and FDA-active fungal hyphal lengths, total bacteria (0–15 cm) and in situ soil respiration rates were measured at approximately monthly intervals. Soil carbohydrates and water-stable aggregate (WSA) distributions were quantified on the final sample date. Surface soil (0–5 cm) of NT had more macroaggregates (> 250 μm diam) and 1.30 to 1.46 times higher densities of fungal mycelia as compared to CT soils. The higher populations of fungal mycelia corresponded to a nearly two-fold higher concentration of acid-hydrolysable carbohydrates, which were composed of proportionally more microbial- than plant-derived sugars. Treatment with the fungicide resulted in a 40% reduction in > 2000 μm WSA and lower concentrations of carbohydrates in NT surface soils, but had no significant effects in CT soils. The contributions of fungi to aggregate stability may represent an important biotically-regulated mechanism for the protection of soil organic matter and may help to explain the greater retention of soil organic carbon in NT than in CT soils.

Physical protection of soil organic S studied using acetylacetone extraction at various intensities of ultrasonic dispersion

Soil structure can protect soil organic matter against microbial attack by physical protection within stable soil aggregates. We have evaluated a technique for extracting organic matter from aggregates with different stabilities. Organic bonded-S, organic bonded-P and some polyvalent metals (Mg, Mn, Ca, Fe, Al, Zn and Cu) were measured in aqueous acetylacetone extracts of a native and a cultivated soil subjected to different intensities of ultrasonic dispersion during extraction. Increasing ultrasonic energy input resulted in increased extraction of organic S, organic P and polyvalent metals, until full dispersion was achieved. Dispersion of the cultivated soil required less energy input than dispersion of the native, uncultivated soil. This is consistent with a lower organic matter content resulting in lower aggregate stability. Polyvalent metals are believed to be responsible for the stabilization of aggregates by forming clay-polyvalent metal-organic matter complexes. In our experiment Mn, Ca, Zn and Cu were only related to stabilization of sand-size aggregates, whereas Al, Fe and Mg were also related to the stabilization of microaggregates. Extraction of S was increased 2.0 and 1.3 times with dispersion of the native and the cultivated soils, respectively, showing that a major part of soil organic S is intimately associated with microaggregates. For both soils, a residual S pool of insoluble organic matter comprised about 50% of total organic S. It is suggested that this extraction technique effectively divides soil organic S into non-protected, protected and insoluble organic S.

Aggregate‐Protected and Unprotected Organic Matter Pools in Conventional‐ and No‐Tillage Soils

AbstractNo‐tillage (NT) practices can result in greater soil aggregation and higher soil organic matter (SOM) levels than conventional‐tillage (CT) practices, but the mechanisms for these effects are poorly known. Our objectives were to describe the size and quality of biologically active pools of aggregate‐associated SOM in long‐term CT and NT soils of the southeastern USA. Samples were collected from replicated CT and NT plots on a Hiwassee sandy clay loam (clayey, kaolinitic, thermic Rhodic Kanhapludult) and separated into four aggregate size classes (&gt;2000, 250–2000, 106–250, 53–106 µm) by wet sieving. Potentially mineralizable C and N and N2O emissions were measured from 20‐d laboratory incuhations of intact and crushed macroaggregates (&gt;250 µm) and intact microaggregates (&lt;250 µm). Three primary pools of aggregate‐associated SOM were quantified: unprotected, protected, and resistant C and N. Aggregate‐unprotected pools of SOM were 21 to 65% higher in surface soils of NT than of CT, with greater differences in the macroaggregate size classes. Disruption of macroaggregates increased the mineralization of SOM in NT but had little effect in CT. Rates of mineralization from protected and unprotected pools of C were higher in surface soils of CT than of NT. Macroaggregate‐protected SOM accounted for 18.8 and 19.1% of the total mineralizable C and N (0–15 cm), respectively, in NT but only 10.2 and 5.4% of the total mineralizable C and N in CT. Our results indicate that macroaggregates in NT soils provide an important mechanism for the protection of SOM that may otherwise be mineralized under CT practices.

Rapid stabilization and mobilization of 15N in forest and range soils

Effects of physical protection of soil organic matter (SOM) on rates of nitrogen (N) mineralization and immobilization were studied in five soils of contrasting mineralogy and physical structure from North and Central America. Whole soils were exposed for 60 days to 15N NH 4C1, then separated into a heavy and a light density fraction ( ≷ 1.70 Mg m −3 ). Substantial 15N was incorporated into the denser (heavy) fraction of each soil during the 60-day exposure indicating that organo-mineral complexes can form quite quickly. Next, sonicated and unsonicated heavy fraction (HF) material was slurry incubated for 5 days. (The effect of sonication was used as an operational measure of the degree of physical protection.) Most of the newly-formed organic N was non-labile. However, some 14N and 15N were released into solution during the slurry incubation, mainly as dissolved organic N (DON). NH + 4 was also released and was more highly labelled than the DON, suggesting that the NH + 4 was derived from a more active pool. We expected that the degree of labelling of the NH + 4 and DON released during incubation would be lower for sonicated than for unsonicated HFs because the label would be diluted by release of physically-protected native 14N. In fact, sonication had no effect on release of 14NH + 4 or 15NH + 4 during the slurry incubation but markedly increased release of 14N and 15N as DON. In addition, sonication increased the degree of labelling of the NH + 4 and perhaps of the DON. Overall these results suggest that some 15N was incorporated into the HFs as an active but protected (“active-protected”) fraction. Only a small portion of this active-protected fraction was chloroform-labile suggesting that it was not microbial biomass.