Organic matter turnover in soil physical fractions following woody plant invasion of grassland:: Evidence from natural 13C and 15N

Soil physical structure causes differential accessibility of soil organic carbon (SOC) to decomposer organisms and is an important determinant of SOC storage and turnover. Techniques for physical fractionation of soil organic matter in conjunction with isotopic analyses (delta(13)C, delta(15)N) of those soil fractions have been used previously to (a) determine where organic C is stored relative to aggregate structure, (b) identify sources of SOC, (c) quantify turnover rates of SOC in specific soil fractions, and (d) evaluate organic matter quality. We used these two complementary approaches to characterize soil C storage and dynamics in the Rio Grande Plains of southern Texas where C(3) trees/shrubs (delta(13)C = -27 parts per thousand) have largely replaced C(4) grasslands (delta(13)C = -14 parts per thousand) over the past 100-200 years. Using a chronosequence approach, soils were collected from remnant grasslands (Time 0) and from woody plant stands ranging in age from 10 to 130 years. We separated soil organic matter into specific size/density fractions and determined their C and N concentrations and natural delta(13)C and delta(15)N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their delta(13)C with time after woody encroachment. The shortest MRTs (average = 30 years) were associated with all particulate organic matter (POM) fractions not protected within aggregates. Fine POM (53-250 mu m) within macro- and microaggregates was relatively more protected from decay, with an average MRT of 60 years. All silt + clay fractions had the longest MRTs (average = 360 years) regardless of whether they were found inside or outside of aggregate structure. delta(15)N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher delta(15)N values reflect an increased degree of humification. Increased soil C and N pools in wooded areas were due to both the retention of older C(4)-derived organic matter by protection within microaggregates and association with silt + clay, and the accumulation of new C(3)-derived organic matter in macroaggregates and POM fractions. (c) 2006 Elsevier Ltd. All rights reserved.