Effect of Wood Ash and Compost Application on Nitrogen Transformations and Availability in Soil-Plant Systems

Application of organic-inorganic by-products, that is, wood ash (WA), compost, and organic manures is a valuable restoration strategy that can alleviate the physical conditions and improve the nutrient status of degraded soils. To get the maximum benefits, proper management and N turnover of these resources need to be examined. The aim of this study was to investigate the effects of WA and compost applied alone or with N fertilizer on different fractions of mineral N, that is, mineralization, nitrification, and recovery of N in soil-pant systems. Laboratory and greenhouse experiments were conducted at the University of Poonch, Rawalakot Azad Jammu and Kashmir, Pakistan. In the incubation experiment, soil (Humic Lithic Eutrudepts, Inceptosols) from an arable field was collected (0-15 cm) and amended with WA, compost, and N fertilizer diammonium phosphate (DAP) [(NH4)(2)HPO4]. In addition, a pot experiment was conducted in the greenhouse to examine the response of sorghum [Sorghum bicolor (L.) Moench] to these amendments. Results indicated that WA and compost significantly increased N mineralization by releasing a maximum of 48.5 and 76.1 mg N kg(-1), respectively compared to 21.9 mg N kg(-1) in the control. Combined application of 1/2DAP with WA and compost resulted in a significant reduction in the extent of total mineral nitrogen (TMN) disappearance observed in N fertilizer (DAP, full) treatment. Both WA and compost significantly increased nitrification potential and accumulation of NO3--N at the end was 36.4 and 70.5 mg kg(-1), respectively compared to 10.3 mg kg(-1) in the control. Wood ash applied alone or in mixtures significantly increased soil pH by 0.99 to 1.45 units. Results obtained from the greenhouse experiment indicated that WA alone did not affect most of the growth characteristics, however, N contents and N uptake by sorghum were significantly increased by WA and compost. Plant N uptake in the combined treatments (1/2DAP+compost, 1/2DAP+WA+compost) was 44 and 57 mg plant(-1), respectively, significantly higher than the 31 mg N plant(-1) in the full DAP treatment. The increase in N uptake and plant dry matter yield was associated with N released from added amendments and a significant correlation existed between TMN and plant N uptake (R-2 = 0.56) and TMN and plant dry matter yield (R-2 = 0.61). The present study demonstrates the existence of substantial amount of N reserve present in these amendments. After proper combinations and management, N from these resources can be taken into account as potential source for the management of nutrient poor soils and plant growth.