Compost inputs, cropping system, and rotation phase drive aggregate-associated carbon

In the central Great Plains of North America, winter wheat (Triticum aestivum L.) is often rotated with fallow periods. Tillage and the reduced return of C inputs have led to extensive soil degradation and loss of soil organic matter. Composted beef manure has the potential to restore soil organic matter and soil function. The aim of this study was to determine the effects of compost application rate and grain vs. forage rotations on soil structure and aggregate-associated C stabilization. The study assessed two dryland rotations of wheat-fallow and triticale (xTriticosecale Wittm. ex A. Camus)/pea (Pisum sativum L.)-fallow, and three compost rates: a control with nothing added (0x), 22.9 t ha(-1) (1x), and 108.7 t ha(-1) (5x) applied every 2 yr. After 8 yr, total C inputs were positively correlated with total soil organic C, the proportion of macroaggregates in the whole soil, and the mean weight diameter of soil aggregates. The increased C storage in macroaggregates was associated with an increase in the particulate organic matter and microaggregates within macroaggregates. Soils receiving the 5x compost rate had increased mid-infrared absorbance at the amide II band, in agreement with the decreased soil C/N ratio. Aggregate fractions of the wheat-based system tended to be more enriched in C than those of the forage-based rotation. Additionally, soil coming out of fallow tended to have aggregate fractions that were depleted in soil C. Our findings indicate that compost can improve soil structure and aggregate-associated C stabilization in dryland agroecosystems, whereas cropping system and rotation phase are also important to consider in the long-term management of SOM.