Estimating carbon retention in soils amended with composted beef cattle manure

Composted cattle manure is often used as a soil amendment to replenish nutrient pools and to supply a source of stable C. Compost composition affects the availability of nutrients and the stability of C following the addition of compost to soil. We investigated C mineralization in a loamy sand and a loam soil amended with nine composts, two fresh manures and alfalfa (Medicago sativa L.) hay at a target rate of 10 mg total C g(-1) soil. Soils were incubated at 25 degrees C for 168 d. There was a significant interaction between amendment and soil type on C mineralization but generally, the effect of soil texture on amendment decomposition was small. The composts were very dissimilar in composition and resulted in substantial differences in the amount of C retained in the soils (2-39% C added evolved as CO2). Total C evolved during the incubation period could be predicted from the NH4-N content and the NH4-N/NO3-N ratio of the composted manures (R-2 = 0.91-0.93). Estimation of the C retained in soils amended with compost as a function of simple chemical properties of the compost provides an important tool for evaluating the effectiveness of compost as a soil amendment, helping to calculate net retention of C.