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,