Effects of cover crops, compost, and manure amendments on soil microbial community structure in tomato production systems
Introduction
Conventional tomato production utilizing tillage, black plastic mulch, commercial fertilizer applied through drip irrigation, and pesticides, can improve the yield and quality of fresh-market tomatoes compared to bare soil production by warming soil earlier in spring, preventing evaporation of soil moisture, increasing fertilizer use efficiency, and suppressing weeds (Abdul-Baki et al., 1992). However, this intensive production system also can degrade soil quality, enhance runoff by covering the soil with an impervious surface, contribute to surface and groundwater pollution, and add to production cost (Rice et al., 2001). Alternative systems have been developed that use renewable organic resources and/or minimize tillage to build soil organic matter and improve soil quality. Minimum-tillage systems including cover cropping with legumes to fix nitrogen, recycle nutrients, and improve soil quality have been identified as viable alternatives for production of both agronomic and horticultural crops (Decker et al., 1994, Lu et al., 2000) and have distinct economic and environmental advantages over use of plasticulture or bare soil (Kelly et al., 1995, Rice et al., 2001).
In a system developed at the USDA Beltsville Agricultural Research Center (Abdul-Baki and Teasdale, 1997), raised beds are formed in the fall and seeded with hairy vetch. The vetch is mowed the following spring just before tomatoes are transplanted through the vetch residue without tillage. The hairy vetch mulch suppresses many annual weeds and provides much of the nitrogen needed by the tomato plants. This surface mulch also reduces raindrop impact on the soil which increases infiltration, reduces runoff and sediment losses from fields, and reduces splashing that can enhance development of foliar diseases (Mills et al., 2002). While tomatoes grown in black plastic mulch produce greater early plant growth and higher early yields, tomatoes grown with hairy vetch mulch have greater overall leaf area, higher total yields, and a longer season (Teasdale and Abdul-Baki, 1997).
Manure and compost are organic sources of nutrients that also have been shown to increase soil organic matter and improve soil quality (Wright et al., 1998). They usually need to be incorporated into soils to prevent loss of nutrients through volatilization or runoff, particularly nitrogen. Thus, they would be suitable for use in conjunction with a traditional tillage-based plasticulture system but could offer potential soil quality improvement in this otherwise soil-destructive system. This approach would provide an easier alternative system for growers to adopt than the minimum-tillage cover crop system that requires new equipment and additional fall operations.
Microbial communities perform essential ecosystem services, including nutrient cycling, pathogen suppression, stabilization of soil aggregates, and degradation of xenobiotics. Soil microbial biomass, activity, and community structure have been shown to respond to agricultural management systems. Soil microbial communities can reflect the impact of management on soil quality although the linkage between soil microbial communities and soil quality is not well understood. Since many soil properties associated with soil quality do not change rapidly in response to management, we evaluated soil microbial communities which would be expected to respond more rapidly to alternative management systems in a short study such as the 2-year experiment presented in this paper (Martini et al., 2004). In this study, we compare soil microbial communities in conventional plasticulture tomato production systems with and without organic soil amendments and in the alternative cover crop-based system. The following hypotheses were tested: (1) hairy vetch cover cropping increases crop yield and significantly affects soil microbial community structure when compared to the standard plastic mulch and synthetic fertilizer-based system; (2) within plastic mulch systems, organic amendments will increase crop yield and significantly affect soil microbial community structure when compared to synthetic fertilizer; (3) crop yields and microbial community structure will be similar in the hairy vetch cover cropping and the organic amended plasticulture systems.
Section snippets
Field experiment
The experiment consisted of eight soil amendment systems for growing fresh-market tomatoes conducted on Downer-Ingleside loamy sand on the North Farm of the Beltsville Agricultural Research Center, Beltsville, Maryland, during the summer seasons of 2000 and 2001. The treatments were synthetic N only (control), three levels of poultry manure compost (5, 10, and 20 Mg/ha), two levels of poultry manure (2.5 and 5 Mg/ha), hairy vetch mulch, and (in 2001 only) hairy vetch mulch plus poultry manure
Tomato yields
In 2000, tomato yields were significantly higher with the highest level of poultry manure than the fertilizer control (Fig. 1). The yields from compost at the highest rate and the hairy vetch treatment were similar to yield of manure at the high rate. In 2001, yields were lower than in 2000 (Fig. 1), probably because of higher foliar disease pressure. Excessive rainfall during the month of July (154 mm) provided conditions highly conducive to development of foliar diseases and led to early
Discussion
Although fertilizer nitrogen rates were lowered to compensate for nitrogen mineralization from the manure, compost, and vetch residue, these treatments still improved yields in 2000, although only when manure and compost were applied at the highest rates tested. This may be because mineralization of manure and compost was enhanced by higher temperatures under black plastic mulch and vetch residue mineralization was enhanced by a consistent, well-distributed rainfall pattern following planting
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