Effects of Tillage and Sampling Depth on the Distribution of Phosphorus and Nitrogen Forms in Manure applied Soils.

Estimating N and P forms in environmental and agricultural soils can be influenced by tillage practices and sampling depths. Under no-till (NT) practices, nutrients mostly concentrate at the soil surface while conventional tillage (CT) distributes nutrients to a greater depth. Soil sampling depth used to estimate nutrients is about 15 cm because most plant roots grow to that depth. Under CT most nutrients are mixed into the soil profile to about 15 cm depth. However, when sampling for soil nitrates, a sample down to a depth of 30 cm may provide a more accurate indication of the amount of nitrate available to the crops, since nitrate is normally leached more easily with soil water than most nutrients. Sampling at shallow depth may over estimate N and P availability likewise sampling to greater depth may underestimate N and P availability. Our objective was to investigate the effects of tillage and sampling depths (0-5, 0-15, and 5-15 cm) on N and P distribution in manure applied soils. Soils were sampled from no-tilled with manure (NTM) and conventionally tilled with manure (CTM) applications plots. Phosphorus forms from soils collected at different depths were extracted using the chemical fractionation scheme of Golterman (1996) and Golterman et al. (1998). Ammonium and nitrate were extracted using 2 M KCl. Result showed that Pi was mostly Fe-bound while Po was mostly organic matter bound at the 0-5 and 0-15 cm depth for NTM. Ammonium nitrogen was highest at the 0-5 cm than at 0-15 cm and 5-15 cm depth for NTM, but no difference was observed with CTM soils. Nitrate nitrogen (NO3-) concentration was also highest at the 0-5 than 0-15 and 5-15 cm depth in the NTM soils. In the CTM plots, NO3- concentration was lowest at 0-5 while 5-15 cm had the highest NO3- concentrations.