Effect of Nitrogen- Vs Phosphorus-Based Manure and Compost Management on Soil Quality.

Appropriate manure management is a critical element of sustainable dairy production.  Current federal and state environmental regulations may increase producer interest in shifting from N-based to P-based manure application. Our objective is to evaluate the impact of a shift from N-based manure applications without incorporation to P-based manure applications with immediate incorporation on soil health factors including soil penetrability (0-10, 10-20, 20-30 and 30-40 cm depth), soil test P (STP), soil test K (STK), organic carbon (C) and overall soil health (Haney test, SHH). Ten treatments implemented in the corn years included six inorganic side-dress nitrogen (N) treatments (0, 56, 112, 168, 224, and 280 kg N ha^-1), N- and P-based liquid dairy manure (178 and 80 kL ha^-1, respectively, averaged over all application years), and N- and P-based composted solids (82 Mg ha^-1 and 40 Mg ha^-1) applied just prior to planting in each corn year. No further nutrient addition took place under the alfalfa years with the exception of 90 kg ha^-1 0-25-25 (N-P₂O₅-K₂O) at establishment. Soil samples and penetrability measurements were taken in spring 2015, 15^th year of a 5-yr corn and 5-yr alfalfa rotation. Shifting from N-based to P-based manure and compost management did not impact soil penetrability. A shift from N-based to P-based compost and manure reduced STP levels by 32 and 48%, respectively. Soil test K levels decreased from 49 to 29 mg kg^-1 and from 35 to 25 mg kg^-1 by shifting from N-based to P-based manure and compost management, respectively. A shift from N-based to P-based applications also resulted in a 20% decrease in organic C where compost was used but did not impact organic C when manure was the source of fertility. Organic C was greater in compost plots than manure and inorganic plots (averaged over two rates) (P≤0.001). Organic N was 17 and 14% higher with N-based treatments than P-based treatments for manure and compost, respectively, and greater in compost treatments than manure treatments (averaged over two rates) (P≤0.001). The overall SHH remained unchanged with a shift from N-based to P-based treatments. However, compost addition resulted in a greater SHH than the manure and inorganic N treatments (P≤0.001) consistent with the higher organic C content upon compost addition and organic C explained 43% of the variability in SHH among treatments. Our results to date show the benefits of compost application for building soil organic matter, the benefits of a shift from N- to P-based application rates for STP and STK, and suggest no measurable impact on aggregate distribution and penetrability. Additional research includes assessments of soil bulk density, aggregate distribution and stability, and greenhouse gas emissions.