Long‐Term Effects of Compost and Cover Crops on Soil Phosphorus in Two California Agroecosystems

Inefficient P use in agriculture results in soil P accumulation and losses to surrounding ecosystems, highlighting the need to use P inputs more efficiently. Composts reduce the need for mineral fertilizers by recycling P from wastes at the regional scale, whereas cover crops reduce soil P losses and have the potential to increase internal soil P recycling by mobilizing soil P “fixed” from previous P applications. We studied the effects of compost and cover crops on soil P in two California experiments, using one to measure the effects of a single cover crop mixture and composted poultry manure across different management practices, and the second experiment to evaluate how different cover crops (pure grass, pure mustard, or grass‐legumes) and yard compost affected soil P dynamics under organic management. We determined changes in soil P dynamics 8 to 18 yr after long‐term experiments were established by measuring chloroform‐extractable P, P sorption capacity, P saturation, and Hedley fractions. Cover crops generally increased microbial and organic P, whereas amendment with yard compost increased resin, microbial, and organic P, with no impact of cover crops and yard compost on other pools or on P sorption. In contrast, addition of composted poultry manure significantly increased all soil P pools (microbial, organic, and inorganic) and P saturation. Our results suggest a limited, moderate, and strong role of cover crops, yard compost, and composted poultry manure, respectively, in affecting soil P in California agroecosystems.