Long-Term Manure Applications Improve Soil Productivity and Sustain High Crop Yield for Acidic Red Soils.

Intensive use of chemical nitrogen (N) fertilizers has resulted in severely reduced productivity of red soils (Ferralic Cambisol) due to accelerated acidification. Manure has been shown to be effective in improving soil productivity by preventing or reversing the acidification process, but little information is available on the required amount in field management. The aim of this research was to determine the effects of various combinations of manure with urea on soil acidification, crop yield, and N use efficiency (NUE) in an 8-year (2009-2016) maize field experiment on a red soil in southern China. Treatments included chemical N, P, and K fertilization (NPK) only, and NPK plus swine manure, which provided 20% (NPKM1), 40% (NPKM2), and 60% (NPKM3) of total N supply (225 kg N ha⁻¹ year⁻¹). Soil pH, exchangeable acidity or base cations, and maize yield or NUE were determined annually. By 2016, the NPK and NPKM1 treatments further reduced soil pH from 4.81 to 4.46 and 4.62, increased soil acidity (dominated by Al³⁺) by 3.24 and 1.48 cmol₍₊₎ kg^-1, decreased base cations, and reduced maize yield to 2,433 and 4,815 kg ha^-1, respectively. In contrast, at the 40% or 60% manure rate (NPKM2 or NPKM3), there were either no change or a significant increase in soil pH (to 5.47), increased base cations, and higher yields (5,193 and 5,411 kg ha^-1, respectively). The NUE from NPKM2 (44.7%) and NPKM3 (46.5%) was significantly higher than that from the NPK (20.6%) and NPKM1 (38.6%) treatments. We conclude that manure incorporation at the annual rate to provide 40% or higher of total N can sustain soil productivity and high crop yield by mitigating acidification in the red soils.