Nitrogen and Carbon Mineralization Dynamics of Manures and Composts

Nitrogen and carbon mineralization rates of 19 manure and compost samples were determined in 1996, with an additional 12 samples evaluated in 1997. These organic amendments were mixed with a soil : sand blend at 2% by dry weight and the amended blends were incubated at constant moisture for 12 (1996) or 24 weeks (1997) at 25 °C. Net N mineralization was measured at 4(1996) or 8-week (1997) intervals, C mineralization at 4-week intervals in 1997. Pots of the amended blends were also seeded with fescue (Festuca arundinacea Shreb.) and watered, but not fertilized, for 17 (1996) or 18 weeks (1997); N phytoavailability was estimated from fescue biomass N and mineral N in pot leachate. An average of 16%, 7%, and 1% of organic N was mineralized in 12 weeks of incubation in 1996, and an average of 15%, 6%, and 2% in 24 weeks of incubation in 1997, in manure, manure compost, and plant residue compost, respectively. Overall, N recovery in the fescue assay averaged 11%, 6%, and 2% of total amendment N for manure, manure compost, and plant residue compost, respectively. Mineralization of manure C averaged 35% of initial C content in 24 weeks, while compost C mineralization averaged only 14%. Within 4 (compost) or 16 weeks (manure), the rate of mineralization of amendment C had declined to a level similar to that of the soil organic C. The use of compost and manure as soil amendments, long a standard practice for organic farmers, is growing in popularity with conventional vegetable farmers in California and elsewhere. This increased usage is fueled in part by recognition that conventional cropping patterns and the intensive tillage typically employed in vegetable production have resulted in loss of soil organic matter. Another factor in many areas is the increased supply of compost because of legislatively mandated recycling of urban yard and landscape wastes (Warnert, 1996). In order to obtain maximum economic value from these organic amendments, and to minimize NO3-N pollution potential, N management practices must be adjusted for expected N mineralization from the amendment. Many studies have evaluated N mineralization rates of various animal manures (Smith and Peterson, 1982). Bitzer and Sims (1988) reported that N mineralization from 20 poultry manures averaged 66% of organic N over a 140-d incubation, but rates varied widely among manures. Douglas and Magdoff (1991) found N mineralization from cow and horse manure much slower, varying from ≈15% of organic N to net N immobilization over a 67d laboratory incubation. Chae and Tabatabai (1986) and Castellanos and Pratt (1981) showed that N mineralization rate varied widely among manures of various types. In a 140-d greenhouse study, Castellanos and Pratt reported rates varying from 57% of organic N for chicken manure, to only 17% for dairy manure. Chae and Tabatabai found similar results in a 26from compost vary nearly as widely as those for noncomposted manures. Castellanos and Pratt (1981) found rates ranging from 4% to 35% in a 10-month assay for manure composts, while Douglas and Magdoff (1991) reported that three manure composts had net N immobilization over a 67-d incubation. Buchanan and Gliessman (1991) found N recovery in a broccoli (Brassica oleracea L. Italica Group) crop to be as high as 57% of applied compost N. Murillo et al. (1995) and Iglesias-Jimenez and Alvarez (1993) reported 22% and 21% N mineralization from urban waste compost, respectively, over a 6-month period. However, Hartz et al. (1996) and Hartz and Giannini (1998) found short-term N immobilization to be common with urban green waste composts, and net mineralization of only 1% to 3% in a 4-month assay. The primary objective of this study was to determine the N mineralization dynamics of a range of manures and composts representative of those currently used as soil amendments in California vegetable production. Concurrent measurement of carbon mineralization provided insight on the effect of these amendments on soil microbial activity, and on soil organic matter maintenance. Materials and Methods Samples of 31 organic amendments intended for agricultural use were collected from various sources throughout central and southern California, 19 in 1996 and 12 in 1997 week laboratory incubation, with chicken manure mineralizing 53% compared to only 31% for horse manure. Composting tends to reduce N mineralization rate of organic wastes (manures, crop residues, etc.), but reported mineralization rates Table 1. Selected characteristics of the organic amendments. Amendment no. Description Total N Organic N P K C C/N Ratio ----------------------g·kg -------------------