Greater soil C inputs accelerate loss of C in cropping systems with low N input

BACKGROUND AND AIMS: Managing soil organic matter (SOM) levels in agricultural systems has focused predominantly on the quantity of plant residues returned to the soil but residue quality may also affect SOM stores and dynamics. Our objective in this research was to evaluate the influence of crop rotation on SOM storage and dynamics in a long-term field experiment using particle size fractionation and natural abundance ¹³C. METHODS: Soils were collected from an 18-year maize (Zea mays L.) and soybean (Glycine max (L) Merr.) cropping experiment that imposed a natural shift in the C isotope ratio of SOM with no addition of fertilizer nitrogen (N). We fractionated soils from time zero and year 18 into three size fractions (>53 μm, 5–53 μm, <5 μm), analyzed the whole soils and fractions for their elemental concentrations and C isotope ratios, and calculated the storage and turnover of C and N in the soil. RESULTS: Soil C and N levels declined in all cropping treatments over time. The quantity and quality of residues returned to the soil over the experimental period differed among the rotation treatments but there was no rotation effect on the storage of C and N in the whole soil or any fraction. The rate of soil C and N loss was positively related to the quantity of residues returned to the soil, suggesting that in this C- and N-limited system, residue addition stimulated decomposition of recalcitrant C. CONCLUSIONS: This study indicates that C inputs alone may not be sufficient for increasing SOC stores and that the availability of nutrients in the plant-soil system must also be considered, particularly when N inputs are limiting. Our results suggest that in a C- and N-limited system, additions of fresh residues may stimulate the microbial community to mine recalcitrant stores of SOM for N, thereby resulting in losses of otherwise stable SOM stores by priming.