Use of 13C and 15N natural abundance techniques to characterize carbon and nitrogen dynamics in composting and in compost-amended soils

Isotope fractionation during composting may produce organic materials with a more homogenous δ13C and δ15N signature allowing study of their fate in soil. To verify this, C, N, δ13C and δ15N content were monitored during nine months covered (thermophilic; >40 °C) composting of corn silage (CSC). The C concentration reduced from 10.34 to 1.73 g C (g ash)−1, or 83.3%, during composting. Nitrogen losses comprised 28.4% of initial N content. Compost δ13C values became slightly depleted and increasingly uniform (from −12.8±0.6‰ to −14.1±0.0‰) with composting. Compost δ15N values (0.3±1.3 to 8.2±0.4‰) increased with a similar reduced isotope variability.The fate of C and N of diverse composts in soil was subsequently examined. C, N, δ13C, δ15N content of whole soil (0–5 cm), light (<1.7 g cm−3) and heavy (>1.7 g cm−3) fraction, and (250–2000 μm; 53–250 μm and <53 μm) size separates, were characterized. Measurements took place one and two years following surface application of CSC, dairy manure compost (DMC), sewage sludge compost (SSLC), and liquid dairy manure (DM) to a temperate (C3) grassland soil. The δ13C values and total C applied (Mg C ha−1) were DM (−27.3‰; 2.9); DMC (−26.6‰; 10.0); SSLC (−25.9‰; 10.9) and CSC (−14.0‰; 4.6 and 9.2). The δ13C of un-amended soil exhibited low spatial (−28.0‰±0.2; n=96) and temporal (±0.1‰) variability. All C4 (CSC) and C3 (DMC; SSLC) composts, except C3 manure (DM), significantly modified bulk soil δ13C and δ15N. Estimates of retention of compost C in soil by carbon balance were less sensitive than those calculated by C isotope techniques. One and two years after application, 95 and 89% (CSC), 75 and 63% (SSLC) and 88 and 42% (DMC) of applied compost C remained in the soil, with the majority (80–90%) found in particulate (>53 μm) and light fractions. However, C4 compost (CSC) was readily detectable (12% of compost C remaining) in mineral (<53 μm) fractions. The δ15N-enriched N of compost supported interpretation of δ13C data. We can conclude that composts are highly recalcitrant with prolonged C storage in non-mineral soil fractions. The sensitivity of the natural abundance tracer technique to characterize their fate in soil improves during composting, as a more homogeneous C isotope signature develops, in addition to the relatively large amounts of stable C applied in composts.