Striking a balance between N sources: Mitigating soil acidification and accumulation of phosphorous and heavy metals from manure

Manure amendment has been shown to effectively prevent red soil (Ferralic Cambisol) acidification from chemicalnitrogen (N) fertilization. However, information is lacking on how much manure is needed to mitigate acidificationand maintain soil productivity while preventing accumulation of other nutrients and heavy metals from long-terminputs. This study determined the effects of various combinations of manure with urea-N on acidification andchanges in soil P, K, and heavy metals in a 9-year maizefield experiment in southern China. Treatments includedchemical N, P and K fertilization only (NPKM0), and NPKplus swine manure, which supplied 20% (NPKM20), 40%(NPKM40), and 60% (NPKM60) of total N at 225 kg N ha−1year−1. Soil pH, exchangeable acidity, available P andK, and maize yield were determined annually from 2009 to 2018. Soil exchangeable base cations, total andphytoavailable Cr, Pb, As, Ni, Cd, Cu, and Zn were measured in 2018. A significant decrease in soil pH occurredunder NPKM0 and NPKM20 from initial 4.93 to 4.46 and 4.71, respectively. Whereas, under NPKM40 andNPKM60 no change or a significant increase in soil pH (to 5.47) occurred, aswell as increased exchangeable base cat-ions, and increased yields. Manure application markedly increased soil available P (but not K) to 67.6–182.6 mg kg−1and significantly increased total Pb, Cu, and Zn and available Cu and Zn in soil. The results indicate sourcing 40% orgreater of total N from manure can prevent or reverse acidification of red soil, and provide all P required, however,additional K inputs are required for balanced plant nutrient supply. An integrated approach of increasing N use effi-ciency, reducing chemical input, and reducing heavy metal concentrations in animal feed are all necessary for sus-tainable use of manure in soil acidity and nutrient management as well as minimizing environmental risks.