Organic carbon and nitrogen in soils of semiarid Argentina

ABSTRACT:

Wind erosion is an important soil degradation process in semiarid environments that can affect soil organic carbon (C) and nitrogen (N) dynamics. Wind erosion can decrease the proportion of easily erodable <0.84 mm (0.03 in) size fractions (aggregates + individual particles), and increase the non erodable. Tillage and rain can breakdown large aggregates increasing organic C and N losses by wind erosion, and can decrease the proportion of aggregates to sand in coarse size fractions. Therefore the variations of organic C and N in size fractions of 20 A-horizons of Haplustolls and Ustipsamments of the semiarid part of Argentina submitted to three management conditions with increasing tillage and wind erosion levels (VIRGIN < Eragrostis curvula PASTURE < AGRICULTURE) were analyzed. The <2, 2 to 50, 50 to 74, 74 to 100, and 100 to 2000 μm size fractions (aggregates + individual sand particles) were separated by wet sieving, and their content of organic C and N determined. The relative amount of aggregates and sand were analyzed together, in order to asses the effect of tillage on the proportion aggregates to sand within each size fraction and the selection caused by wind erosion. Results showed that AGRICULTURE decreased the 74 to 100 um and the 2 to 50 μm sized fractions in relation to VIRGIN soils. Decreases of 74 to 100 um fractions were attributed to the breakdown by tillage of the aggregates composing this size fraction; while decreases of 2 to 50 μm sized fractions (100 percent aggregates) were attributed to losses by wind erosion. PASTURES showed lower proportions of 2 to 50 μm sized fractions than VIRGIN soils, probably as a consequence of wind erosion events occurred before the Eragrostis curvula pasture plantation. The amount of aggregates of the 100 to 2000 um size fraction did not differ between management systems probably because they suffered a rapid turnover in AGRICULTURE soils. Nevertheless, the proportion of sand in relation to aggregates was higher in comparison to VIRGIN soils in this size fraction, which indicates that a relative accumulation of sand occurred. The 2 to 50 μm size fraction (aggregates) were positively correlated with contents of total organic matter (R² = 0.55, p<0.001) and silt + clay [R² = 0.39, p<0.01]. A multiple regression analysis demonstrated that both variables explained 77 percent of the aggregate variability. Organic C and N contents tend mainly to decrease from fine to coarse aggregates within each management type. This was attributed to the formation of organo-mineral complexes with fine sized particles in finer aggregates. Organic C and N contents of coarse fractions (>50 μm) were higher in VIRGIN than in both PASTURE and AGRICULTURE soils, indicating that cultivation promoted C and N losses in coarser fractions. AGRICULTURE promoted larger N than organic C losses in coarse fractions, indicating that nitrogenous compounds were more affected than C compounds. Organic C and N contents in < 50 sized fractions, (i.e. aggregates) did not differ between management systems, probably because of the higher stability of nitrogenous and C compounds accumulated in these fine fractions as organo-mineral complexes. C/N ratios remained unchanged in most cases, indicating homogeneous composition of organic matter with all management types. We concluded that VIRGIN soils converted to agriculture in the semiarid part of Argentina will decrease the proportion of coarse fractions, mainly by breakdown of their aggregates by tillage, and will increase the proportion of sand in these size fractions. The 2 to 50 μm sized aggregates generated by the breakdown of coarser aggregates will be eroded by wind. The concentration of organic C and N will be drastically decreased by cultivation in coarse size fractions but not in the fine ones. The Eragrostis curvula pasture is not effective in recovering soil aggregation nor increasing organic C or N levels in coarse fractions of degraded soils.