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Although irrigation is considered a beneficiary management for increasing soil organic C (SOC) stocks in (semi)arid enviromnents, our understanding of the impact of irrigation on soil organic matter (SOM) dynamics in the field remains limited. We investigated the effect of irrigation on soil C storage in relation to soil aggregation by measuring C stocks of bulk soil and different aggreagate fractions in the top 20-cm layer of center-pivot irrigated vs. dryland farming systems in semiarid southwestern Nebraska. The irrigated fields (IRR) showed increased C inputs and larger SOC stocks than the dryland cultivated fields (DRY). Fractionation of bulk soil samples into non-microaggregate-associated particulate organic matter (free POM) and microaggregate-associated POM, silt, and clay fractions indicated that the larger bulk SOC stock under IRR was explained solely by an increase in microaggregate-associated C storage. Wet sieving of bulk soil showed that microaggregation was remarkably low under DRY and did not increase under IRR, suggesting that the protection of microaggregates inside macroaggregates was no prerequisite for C sequestration under IRR. The results of this study confirm the potential of irrigation to increase soil C stocks through preferential sequestration of C inside microaggregates, but question our understanding of the mechanisms underlying this preferential sequestration. Abbreviations: cPOM, coarse particulate organic matter (>250 [micro]m); DRY, dryland cultivated treatment; iPOM, intramicroaggregate particulate organic matter; IRR, center-pivot irrigated treatment; mClay, microaggregate-associated clay fraction; micro-C, microaggregate-associated C; mSilt, microaggregate-associated silt fraction; NV, native vegetation; POM, particulate organic matter; SOC, soil organic carbon; SOM, soil organic matter; TOC, total organic carbon.

Identification of diagnostic soil organic matter (SOM) fractions and the mechanisms controlling their formation and turnover is critical for better understanding of C dynamics in soils. Enhanced microaggregate formation and stabilization of C due to reduced macroaggregate turnover has been proposed as a mechanism promoting C sequestration in no-tillage (NT) compared with conventional tillage (CT) systems in temperate soils dominated by 2:1 clay mineralogy. We evaluated the contribution of macroaggregate-protected microaggregates to total soil organic carbon (SOC) sequestration in NT relative to CT in three soils differing in clay mineralogy: a 2:1 clay-dominated soil (2:1), a soil with mixed clay mineralogy [2:1 and 1:1] and oxides (mixed), and a soil dominated by (1:1) clay minerals and oxides (1:1). Microaggregates (mM) were isolated from macroaggregates from 0- to 5- and 5- to 20-cm soil layers. Particulate organic matter (POM) located within the microaggregates (intra-mM-POM) was separated from POM outside of the microaggregates (inter-mMPOM) and the mineral fraction of the microaggregates (mineral-mM). In all three soils, total SOC as well as microaggregate-associated C (mM-C) was greater with NT compared with CT. Although less than half of the total SOC under NT was associated with the microaggregate fraction, more than 90% of the total difference in SOC between NT and CT was explained by the difference in mM-C in all three soils. Thus, we identified and isolated a fraction that explains almost the entire difference in total SOC between NT and CT across soils characterized by drastically different clay mineralogy.

Global circulation models predict shifts in precipitation patterns in arid and semiarid ecosystems worldwide. Independently of changes in total precipitation, rainfall is expected to concentrate in less frequent but larger events, and this redistribution will increase drought severity in biomes naturally controlled by precipitation pulses and water availability. This study focuses on effects of altering rainfall patterns on the soil food web of a semiarid prairie, and on impacts of grazing on soil food web resilience to climate change. We worked in a greenhouse with undisturbed soil monoliths extracted from grazed and ungrazed field plots. We planted all monoliths with Bouteloua gracilis that was the dominant grass at the experimental site, and let the grass establish before applying two different rainfall treatments over a five-month period, equivalent to the duration of the growing season in the area. We simulated the current rainfall pattern in the experimental area and also an altered rainfall pattern consisting of concentrating the total growing season precipitation in half the number of rain events. Rainfall concentration increased the amplitude of the water pulse and lengthened the dry periods between rain events thus worsening soil water stress. We analyzed the experimental monoliths for effects of rainfall pattern and grazing management C and N content, soil organic matter mineralization and soil biota abundance and functional diversity. Based on the experimental data, we simulated soil food web stability and mineralization capacity. We concluded that (a) rainfall concentration causes loss of key soil functional groups and reinforces soil food web stability, (b) organic matter mineralization by the soil food web decreases with rainfall concentration, (c) coarse-textured soils are more responsive than fine-textured soils to modifications of the rainfall regime, (d) soil food web responsiveness to changing rainfall regime is higher in grazed prairies than in prairies excluded from herbivores. [ABSTRACT FROM AUTHOR]

Amino sugars have been used as biomarkers to assess the relative contribution of dead microbial biomass of different functional groups of microorganisms to soil carbon pools. However, little is known about the dynamics of these compounds in soil. The isotopic composition of individual amino sugars can be used as a tool to determine the turnover of these compounds. Methods to determine the delta(13)C of amino sugars using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) have been proposed in literature. However, due to derivatization, the uncertainty on the obtained delta(13)C is too high to be used for natural abundance studies. Therefore, a new high-performance liquid chromatography/isotope ratio mass spectrometry (HPLC/IRMS) methodology, with increased accuracy and precision, has been developed. The repeatability on the obtained delta(13)C values when pure amino sugars were analyzed were not significantly concentration-dependent as long as the injected amount was higher than 1.5 nmol. The delta(13)C value of the same amino sugar spiked to a soil deviated by only 0.3 per thousand from the theoretical value., (2009 John Wiley & Sons, Ltd.)