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SOIL BIOLOGY & BIOCHEMISTRY

Tillage Effects on Microbial and Carbon Dynamics during Plant Residue Decomposition

^a Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506
^b current address: USDA-ARS/SEWRL, 2316 Rainwater Rd., Tifton, GA 31794
^c Dep. of Agronomy, Kansas State Univ., 2004A Throckmorton Plant Sciences Center, Manhattan, KS 66506

^* Corresponding author (Paul.White{at}ars.usda.gov).

One goal of soil C sequestration is to increase the mass of C stored in agricultural soils. Reducing soil disturbance, e.g., no-till management, facilitates soil fungal growth and results in higher C sequestration rates; however, the specific mechanisms associated with short-term plant residue C and N retention are less clear. We applied ¹³C- and ¹⁵N-enriched grain sorghum [Sorghum bicolor (L.) Moench] residue to no-till (NT) and conventional tillage (CT) soils, and measured the ¹³C and ¹⁵N retention in the soil and in aggregate fractions, along with soil microbial dynamics, during a growing season. The added plant residue mineralized rapidly in both tillage systems, with similar decomposition kinetics, as indicated by ¹³C data. Mass balance calculations indicated that approximately 70% of the added ¹³C was mineralized to CO₂ by 40 d. Total Gram positive and Gram negative bacteria and fungal phospholipid fatty acids were higher under NT 0 to 5 cm during the most active period of residue mineralization compared with the CT 0- to 5- or 5- to 15-cm depths. No changes were observed in the NT 5- to 15-cm depth. The >1000-µm aggregate size class retained the most ¹³C, regardless of tillage. The NT >1000-µm aggregates retained more ¹⁵N at the end of the experiment than other NT and CT aggregates size classes. Data obtained indicate higher biological activity associated with NT soils than under CT, and increased retention of plant residue C and N in macroaggregates.

Abbreviations: CT, conventional tillage • Gm⁺, Gram positive • Gm^–, Gram negative • NT, no-till • PLFA, phospholipid fatty acids • R+, residue added • R–, no residue added • SOC, soil organic carbon • TC, total carbon • TN, total nitrogen • VPDB, Vienna PeeDee Belemnite