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

• Published in: Soil biology & biochemistry Vol. 38; no. 11; pp. 3197 - 3210
• Main Authors: Liao, J.D., Boutton, T.W., Jastrow, J.D.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Oxford Elsevier Ltd 01.11.2006; New York, NY Elsevier Science
• Subjects: Agronomy. Soil science and plant productions; Biochemistry and biology; Biological and medical sciences; C3 plants; C4 plants; Carbon cycle; carbon sequestration; Chemical, physicochemical, biochemical and biological properties; Chronosequence; chronosequences; ecological invasion; fractionation; Fundamental and applied biological sciences. Psychology; grassland soils; grasslands; nitrogen; Nitrogen cycle; Organic matter; Organic matter stabilization; Particulate organic matter; Physics, chemistry, biochemistry and biology of agricultural and forest soils; Soil aggregates; soil chemical properties; soil nutrient dynamics; soil organic matter; soil physical properties; Soil science; Stable isotopes; Woody plant invasion; woody plants; Texas; United States; North America; America; Carbon cycle; Chronosequence; Nitrogen cycle; Stable isotopes; Soil aggregates; Organic matter stabilization; Particulate organic matter; Woody plant invasion; Grassland; Organic matter; Isotope labelling; C4-Type; Woody plant; Physical properties; Invasive species; C-13; Organic matter cycle; Nitrogen-15; Soil science; Aggregate; Isotopes; C3-Type; Grassland soil
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2006.04.004

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 (δ 13C, δ 15N) 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 (δ 13C=−27‰) have largely replaced C 4 grasslands (δ 13C=−14‰) 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 δ 13C and δ 15N values. Mean residence times (MRTs) of soil fractions were calculated based on changes in their δ 13C 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 μm) 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. δ 15N values of soil physical fractions were positively correlated with MRTs of the same fractions, suggesting that higher δ 15N 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.