Abstract of KTSOM project

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KTSOM Abstract

Soil Structure and Soil Organic Matter: I. Distribution of Aggregate Size Classes and Aggregate Associated Carbon. J. Six, K. Paustian, E.T. Elliott, and C. Combrink. Soil Science Society of America Journal. In press (2000)

     Cultivation reduces soil C content and changes the distribution and stability of soil aggregates. We investigated the effect of cultivation intensity on aggregate distribution and aggregate C in three soils dominated by 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. Slaked (i.e. air-dried and fast rewetted) and capillary rewetted soils were separated into four aggregate size classes (< 53, 53-250, 250-2000 and > 2000 µm) by wet sieving. In rewetted soils, the proportion of macroaggregates accounted for 85% of the dry soil weight and was similar across management treatments. In contrast, aggregate distribution from slaked soils increasingly shifted towards more microaggregates and fewer macroaggregates with increasing cultivation intensity. In soils dominated by 2:1 clay mineralogy, the C content of macroaggregates was 1.65 times greater compared to microaggregates. These observations support an aggregate hierarchy in which microaggregates are bound together into macroaggregates by organic binding agents in 2:1 clay dominated soils. In the soil with mixed mineralogy, aggregate-C did not increase with increasing aggregate size. At all sites, rewetted macro- and microaggregate C and slaked microaggregate C differed in the order NV > NT > CT. In contrast, slaked macroaggregate C concentration was similar across management treatments, except in the soil with mixed clay mineralogy. We conclude that increasing cultivation intensity leads to a loss of carbon-rich macroaggregates and an increase of carbon-depleted microaggregates in soils that express aggregate hierarchy.

Aggregate and soil organic matter dynamics under conventional and no-tillage systems. J. Six, E.T. Elliott, and K. Paustian. Soil Science Society of America Journal 63, 1350-1358 (1999)

     Tillage generally reduces aggregation and particulate organic matter (POM) content. We hypothesized that reduced C sequestration in conventional tillage (CT) compared to no-tillage (NT) is related to differences in aggregate turnover. Four soils (Haplustoll, Fragiudalf, Hapludalf, and Paleudalf), each with NT, CT and native vegetation (NV) treatments, were separated into aggregates. Free light fraction (free LF) and intraaggregate-POM (iPOM) were isolated. At one site we used ¹³C natural abundance to differentiate crop and grassland-derived C. Concentrations of coarse iPOM-C (250-2000 µm iPOM in macroaggregates), expressed on a per unit aggregate, did not differ between tillage treatments. In contrast, concentrations of fine iPOM-C (53-250 µm iPOM in macroaggregates) were less in CT compared to NT macroaggregates. On a whole soil basis, fine iPOM-C was on average 51% less in CT compared to NT, and accounted for 21% of the total C difference between NT and CT. The concentration of free LF-C was not affected by tillage, but was on average 45% less in the cultivated systems compared to NV. Proportions of crop-derived C in macroaggregates were similar in NT and CT, but were three times greater in microaggregates from NT compared to microaggregates from CT. We suggest that a faster turnover rate of macroaggregates in CT compared to NT leads to a slower rate of microaggregate formation within macroaggregates, and less stabilization of new SOM in free microaggregates under CT.

Soil Structure and Soil Organic Matter: I. Distribution of Aggregate Size Classes and Aggregate Associated Carbon. J. Six, K. Paustian, E.T. Elliott, and C. Combrink. Soil Science Society of America Journal. In press (2000)

     Cultivation reduces soil C content and changes the distribution and stability of soil aggregates. We investigated the effect of cultivation intensity on aggregate distribution and aggregate C in three soils dominated by 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) mineralogy. Each site had native vegetation (NV), no-tillage (NT), and conventional tillage (CT) treatments. Slaked (i.e. air-dried and fast rewetted) and capillary rewetted soils were separated into four aggregate size classes (< 53, 53-250, 250-2000 and > 2000 µm) by wet sieving. In rewetted soils, the proportion of macroaggregates accounted for 85% of the dry soil weight and was similar across management treatments. In contrast, aggregate distribution from slaked soils increasingly shifted towards more microaggregates and fewer macroaggregates with increasing cultivation intensity. In soils dominated by 2:1 clay mineralogy, the C content of macroaggregates was 1.65 times greater compared to microaggregates. These observations support an aggregate hierarchy in which microaggregates are bound together into macroaggregates by organic binding agents in 2:1 clay dominated soils. In the soil with mixed mineralogy, aggregate-C did not increase with increasing aggregate size. At all sites, rewetted macro- and microaggregate C and slaked microaggregate C differed in the order NV > NT > CT. In contrast, slaked macroaggregate C concentration was similar across management treatments, except in the soil with mixed clay mineralogy. We conclude that increasing cultivation intensity leads to a loss of carbon-rich macroaggregates and an increase of carbon-depleted microaggregates in soils that express aggregate hierarchy.

Soil Structure and Soil Organic Matter: II. A Normalized Stability Index and the Effect of Mineralogy. J. Six, E.T. Elliott, and K. Paustian. Soil Science Society of America Journal. In press (2000)

     Soil aggregate distribution and stability measurements have been proposed as soil quality indicators. However, pretreatment of soil, antecedent water content and differences in sand size distribution a mong soils can confound interpretation of these measurements. We propose a normalized stability index (NSI) which (i) compares aggregate distribution after slaking and rewetting to characterize whole soil stability and eliminate confounding effects of pretreatment and antecedent water content, (ii) corrects for the confounding effect of differences in sand size distribution among soils, aggregate size classes and pretreatments, and (iii) normalizes the level of disruption imposed by slaking by using a maximum level of disruption. The NSI was tested on three soils dominated by a 2:1 clay mineralogy and one soil characterized by a mixed (2:1 and 1:1) clay mineralogy. Each site had native vegetation (NV), no-tillage (NT) and conventional tillage (CT) treatments. In soils dominated by 2:1 clays, NSI decreased with increasing cultivation intensity (NV > NT > CT). However, NSI was higher in the soil with mixed clays compared to the other soils and was not different along the cultivation gradient. These observations are hypothesized to be related to the presence of Fe- and Al-oxides and kaolinite. In conclusion, NSI appears to be a good indicator for soil structural quality since it is sensitive to changes in agricultural practices and it minimizes confounding effects. A decrease of SOM levels results in a smaller decrease of soil stability in soils dominated by oxides and 1:1 minerals compared to soils dominated by 2:1 minerals.

The Enriched Labile Soil Organic Matter Fraction: A Reevaluation. J. Six, R. Merckx, K. Kimpe, K. Paustian and E.T. Elliott. European Journal of Soil Science. In press (2000)

     Identifying 'functional' pools of soil organic matter and understanding their response to tillage remains elusive. We have studied the effect of tillage on the enriched labile fraction, thought to derive from microbes and having an intermediate turnover time. Four soils, each under three regimes, long-term arable use without tillage (NT), long-term arable under conventional tillage (CT), and native vegetation (NV) were separated into four aggregate size classes. Particle size fractions of macro- (250-2000 µm) and micro- (53-250 µm) aggregates were isolated by sonication and sieving. Subsequently, densiometric and chemical analyses were conducted on fine-silt-sized (2-20 µm) particles to isolate and identify the enriched labile fraction. Across soils, the amounts of C and N in the particle size fractions were highly variable and were strongly influenced by mineralogy; specifically the contents of Fe and Al oxides. This evidence indicates that the fractionation procedure cannot be standardized across soils. In one soil, C associated with fine-silt-sized particles derived from macroaggregates was 567 g C m^-2 under NV, 541 g C m^-2 under NT, and 135 g C m^-2 under CT, whereas C associated with fine-silt-sized particles derived from microaggregates was 552, 1018, 1302 g C m^-2 in NV, NT, and CT respectively. This and other data indicate that carbon associated with fine-silt-sized particles is not significantly affected by tillage. Its location is simply shifted from macroaggregates to microaggregates with increasing tillage intensity. Natural abundance ¹³C analyses indicated that the enriched labile fraction was the oldest fraction isolated from both macro- and microaggregates. We conclude that the enriched labile fraction is a 'passive' pool of soil organic matter in the soil and is not derived from microbes nor sensitive to cultivation.