Long term effect of various fertilizing strategies on soil organic carbon, crop yield, soil P availability & potential P leaching

In 2005, a field experiment with arable and vegetable crops was started on a sandy loam soil. The experiment compares 8 fertilizer treatments: MIN (mineral fertilizers), FYM (farmyard manure), CSL (cattle slurry), VFG (vegetable, fruit and garden waste compost), CMC1 (farm compost, high C/N), CMC2 (farm compost, low C/N), NF+ (no fertilizer) and NF- (fallow). The slurry addition rate in the CSL treatment is yearly calculated using the N balance method. The other organic fertilizer treatments are normalized for C input (2-3 Mg C/ha.yr) with the CSL treatment. Extra mineral NPK is applied to equalize between treatments for plant available N, 100 kg P2O5/ha.yr and 300 kg K2O/ha.yr. During the course of the field trial crop yields were measured. In 2011 and 2012 TOC (total organic carbon), P availability (0.01M CaCl2), potential plant available P (ammonium lactate, pH 3.75) and P export were measured. In 2011 a soil column laboratory experiment was conducted, to investigate potential P leaching from the top soil (0-30 cm).
For every year the dry matter (DM) crop yield was calculated relative to the mean DM crop yield of all fertilized treatments. Calculated over the entire period (2005-2013), the relative DM crop yield was CSLa (94)<MINa (94)<CMC1ab (100)<FYMab (103)<VFGab (104)<CMC2b (106).
Although C input was equal in all organic treatments, there was a clear effect of carbon source on the TOC content: VFG (1.32%)>CMC2 (1.27%)>FYM (1.24%)>CMC1 (1.21%)>CSL (1.13%)>NF+ (1.05%)>MIN (1.03%)>NF- (0.97%). The P availability in the FYM treatment was clearly higher compared to all other treatments. However, this effect was not always significant. No significant differences were found in potential plant available P and P export between the fertilized treatments.
The leaching experiment revealed a significantly higher (p<0.05) total P leaching potential for the FYM treatment compared to the other treatments. The speciation of the total leached P was not influenced by the fertilizer treatments. Approximately, 80-90% and 45-60% leached as total dissolved P and ortho-P, respectively.
In conclusion, only the application of farm yard manure resulted in enhanced P availability levels, while this did not lead to significantly higher DM crop yield or P export. For the organic fertilizers applied, an increase of the potential P leaching has been observed only for FYM.