Effects of Cover Crops Mulching and Green Manure

Cover and catch crops, mulch, residue, and green manure provide many services to agroecosystems, from erosion and weed control to deposition and protection of organic matter (Jackson 2000) and promotion of genes for disease resistance (Kumar et al. 2004). Some of such services reduce yield of the main crop, especially in intercropping systems and in environments where water and/or nutrients availability are limited. Therefore appropriate management is required, and trade-offs between ecological benefits and production must be approached with solutions that are hardly transferable and need to rely on research conducted in relevant conditions.

Dead mulch, green manure, and crop residues left in the field provide OM to soil, while cover crops also provide OM to soil as exudates in rhizosphere. Gale and Cambardella (2000) and Steenwerth and Belina (2008b) have shown that the contribution of root-C deposition to SOM content and to the consequent stimulation of the microbial metabolism is larger than that produced by soil incorporation of above-ground plant residue. Both living and dead plant cover exhibits a mulching behavior, by reducing soil evaporation, buffering soil temperature (thus reducing SOM oxidation), limiting rainfall impact and transport with runoff, and controlling weeds by soil shading. Uptake of water or nutrients (catch effect) can be only ascribed to plant living cover. More complex effects are linked to pest spread and control, and to other environmental issues such as CO2 and N2O emissions (Alluvione et al. 2010). In regions where soil water is the limiting factor, like in Mediterranean area, large emissions from cover crop soils may be restricted to high precipitation periods (Steenwerth and Belina 2008a).

A body of literature concerning soil in temperate regions, and recently in Mediterranean systems, indicates that indirect cover crop effects on SOM, related to reduced need for tillage and fertilization, are as important as direct deposition (Andrews et al. 2002; Hulugalle et al. 2006; Veenstra et al. 2007; Alvaro-Fuentes et al. 2008; Steenwerth and Belina 2008b). SOM increase, in turn, improves soil structure and stability and leads to nutrient retention. Cover crops improve nutrient use efficiency through many mechanisms: provide N through fixation, prevent N losses by inducing a better synchronization of N mineralization and uptake (Drinkwater 2004), help nutrients transfer from year to year, and release them in soil layers where cash crops can take them up more easily (Thorup-Kristensen et al. 2003).

Soil covering also increases organic N availability for mineralization processes, thus reducing the need for N fertilization (Drinkwater 2004). Microbe-mediated processes are also induced by soil mulching and cover crop soils often show a larger microbial N content and a greater capacity for N mineralization, nitrification, denitrification than tilled soils (Steenwerth and Belina 2008b). Effects on crop yield are highly variable: intercropping may result in competition for water and nutrients - especially N - to the point of requiring additional irrigation or fertilization (Skroch and Shribbs 1986). Teasdale et al. (2007) report cash crop yield reductions due to live cover, and regrowth or residues from cut cover crops.

Hairy vetch is largely used as a fall-winter cover in temperate to Mediterranean environments for its rapid establishment in fall and vigorous growth in early spring before planting a macrothermal cash crop. The advantages reside in efficient weed control due to crop planting operations and thick stand, soil protection from fall-winter precipitation, and nitrogen fixation (Teasdale et al. 2007). However, its adoption need to be studied in more detail since weed suppression before harvest is counter-balanced by weed stimulation exerted by decomposing vetch residues, likely because of nitrogen release, as found for Amaranthus spp. (Teasdale and Pillai 2005). In general, weed suppression from cover crops appears to be linked to appropriate management rather than to direct effects (Barberi and Mazzoncini 2001).

Experiments on herbaceous cover crops were conducted in Basilicata with the aim of optimizing management for soil protection and yield. In general, grass resulted more efficient in reducing erosion than legumes (Postiglione et al. 1989, 1990), but SOM increases were only found in conjunction with zero tillage and only in the top 10 cm of soil. Pastor et al. (2000) reported SOM and nutrients increases by using cover crops or residue mulching in Mediterranean soils, only in the top 2-5 cm. Living herbaceous covers are intercropped in Mediterranean orchards for erosion control, but in late spring, when precipitations decrease, competition for water may cause yield losses (Pastor et al. 2000). Management options include harvesting the cover crop in early-mid spring and using it for forage or green manure, or simply eliminating it chemically (Pastor et al. 2000). However, tradeoffs with reduced soil protection need to be considered with regards to late spring precipitation, and especially to late summer/early fall intense rainstorms, which may be most erosive (Postiglione et al. 1990) especially in absence of soil cover.

Spontaneous living cover was established under olive trees in a 30% sloping soil in Basilicata, and Chiaffitelli et al. (2005) investigated the evolution of potential soil protection after death of cover species in spring. They found that belowground residue decomposition was very low throughout the summer when the soil was dry, and the soil protection from herbaceous root residue was unchanged in early fall. In the same field setting, the lack of N in highly eroded areas favored the colonization of N-fixing cover species, whereas nonfixing species, namely grass, would have been more desirable because of their greater potential for soil protection. The spatial distribution of soil-protecting species was improved after compost application. Chiaffitelli et al. (2005) concluded that managing the complexity introduced by intercropping can enhance the environmental benefits of cover crops and retain practices oriented toward minimizing yield losses. In Mediterranean environments, it was found that the grass cut in spring and left on soil until fall was more effective than N-fixing species (Chiaffitelli et al. 2005), whose residue undergo faster degradation due to low C/N ratios (Pastor et al. 2000). However, low decomposition rates of surface residue in dry summer conditions imply risk of fire (Pastor et al. 2000), while an excess of mulched residues may interfere with seeding of next fall crops and prevent full crop growth due to toxins release, N immobilization, and alteration of soil temperature and aeration (Teasdale et al. 2007).

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