Mulching with composted municipal solid wastes in the Central Negev, Israel II. Effect on available nitrogen and phosphorus and on organic matter in soil

Composts are stabilized organic residues that are expected to decompose slowly in soil. When applied annually, they may increase the load of organic nitrogen that will eventually mineralize, and subsequently nitrates in excess of crop demand will be leached towards ground waters. A 4-year field experiment was conducted on a Xerosol in the Central Negev in Israel, in which composted municipal solid waste (CMSW) was added as mulch each year to conserve water for rain fed crops. Our objective in this study was to evaluate the amount of mineral N released from the CMSW and the buildup of soil organic matter (SOM), in order to minimize the hazard of nitrate contamination under this practice. CMSW was spread over the soil surface (100 and 300 m(3) ha(-1)) each autumn prior to seeding, and in the following year the residual material was incorporated into the soil at seedbed preparation. We measured: (i) emission Of CO2 from the soil surface during 2 months commencing after the first heavy rainstorm. (ii) SOM content below the CMSW mulch twice a year, and (iii) concentrations of available nitrogen and phosphorus 3-4 times during each cropping season. In the 100 m(3) ha(-1) CMSW treatment, SOM increased by 21 % of the organic matter added by CMSW in 3 years, whereas the increase in CO2-C emission due to this CMSW treatment was only 12% of the C applied. Both measured parameters were not proportional to the amount of CMSW applied, thus indicating that the Gap between measured mineralization and Gain of SOM in the treatment of 300 m(3) ha(-1) CMSW was even larger than in 100 m(3) ha(-1). The concentrations of available N and P in soil were sufficient for wheat (Triticum aestivum L.) during its main Growth period, and were also disproportional to the amount of CMSW added. Loss of nitrate from the root zone during the Growth period of wheat was twice the amount of estimated N uptake, if we assumed that only the soluble N of the composts became available. However, the small gain in SOM implied that more N than the soluble N was mineralized and lost, although the distribution of nitrates with depth did not show considerable leaching. Annual mulching with 100 m(3) ha(-1) CMSW, which was adequate for water preservation, provided sufficient available N and P for rain fed wheat. Larger amounts than needed for sufficient mulching could potentially contaminate ground water with excess nitrate. Possibly smaller amounts of CMSW than 100 m(3) ha(-1) should be considered for safe, long-term annual application in light textured soils with a small capacity to retain SOM and in warmer temperatures during the rainy season. (C) 2004 Elsevier B.V. All rights reserved.