Stable Isotope Techniques using Enriched ¹⁵N and ¹³C for Studies of Soil Organic Matter Accumulation and Decomposition in Agricultural Systems

Abstract

Crop residues, particularly from N₂ fixing legumes in rotations, are considered to be critically important in building up soil organic matter, thereby improving sustainability and providing N and other nutrients to following non-legume crops. Studies in this regard have traditionally involved the addition to ^-soil of plant residues labelled with isotopes, and subsequent monitoring of total and labelled fractions through time to assess rates of residue turnover. In most of these studies residues labelled with ¹⁵N and ¹³C, the stable isotopes of nitrogen and carbon, have been used (e.g. Broadbent and Nakashima 1974, Darwis 1993, Aita et al. 1997, Jensen 1997), or alternatively with ¹⁴C, the radio-active isotope of carbon (Amato and Ladd 1980, Jawson et al. 1989, Voroney et al. 1989). However, the use of stable isotopes has been favoured due to increasing safety restrictions governing the use of some radio-tracers coupled with rapid technological advancements in measurement of stable isotopes by continuous flow mass spectrometry (CF-MS). Within this framework incubation studies have provided considerable information concerning the effects of quantity and quality (structural and chemical composition, ratio of C:N) of fresh residue additions on decomposition rates (Franzluebbers et al. 1994, Vanlauwe et al. 1996). Studies have also included investigations of the effects of variable soil conditions including moisture and temperature, and inherent soil chemical and physical properties such as pH and texture, on residue decomposition rate (Clay and Clapp 1990, Ladd et al. 1995, Ladd et al. 1996).