Mixing organic amendments with high and low C/N ratio influences nutrient availability and leaching in sandy soil

Little is known about available N and P, microbial biomass and leachate N and P concentration in mixes of organic materials differing in C/N ratio. Sandy soil was amended with wheat straw (W, C/N 71) and cow manure (CM, C/N 7) either alone (100W and 100CM) or in different ratios (values are weight percentage of the organic materials): 75W-25CM, 50W-50CM, 25W-75CM. The control was unamended soil. Moist soil was incubated for 26 days, leaching was carried out on days 10 and 25. Soil was sampled on day 9 (before first leaching) and day 26 (after the second leaching). Cumulative respiration over 26 days was similar in the unamended control and 100CM, it increased with proportion of W in the amendments. Per kg of soil, amended soils did not differ in available N, microbial biomass N (MBN) and leachate inorganic N concentration, but available P and leachate inorganic P increased with proportion of CM in the amendment. However, available N, MBN and inorganic N in leachate per g N added was highest in 100W and lowest in 100CM. In contrast, available P and leachate P concentration per g P added increased with proportion of CM. Measured available N and leachate inorganic N were lower than expected values whereas measured available P and inorganic P in the leachate were higher than expected. In mixes, CM appears to reduce N mineralisation in wheat whereas W stimulates P release from cow manure.