Cover crops enhance soil organic matter, carbon dynamics and microbiological function in a vineyard agroecosystem

Abstract

Impacts of soil tillage and cover crops on soil carbon (C) dynamics and microbiological function were investigated in a vineyard grown in California's mediterranean climate. We (1) compared soil organic matter (SOM), C dynamics and microbiological activity of two cover crops [Trios 102 (Triticale × Triosecale) (‘Trios’), Merced Rye (Secale cereale) (‘Rye’)] with cultivation (‘Cultivation’) and (2) evaluated seasonal effects of soil temperature, water content, and precipitation on soil C dynamics (0–15 cm depth). From treatments established in November 2001, soils were sampled every 2–3 weeks from November 2005 to November 2006. Gravimetric water content (GWC) reflected winter and spring rainfall. Soil temperature did not differ among treatments, reflecting typical seasonal patterns. Few differences in C dynamics between cover crops existed, but microbial biomass C (MBC), dissolved organic C (DOC), and carbon dioxide (CO₂) efflux in ‘Trios’ and ‘Rye’ were consistently 1.5–4-fold greater than ‘Cultivation’. Cover crops were more effective at adding soil C than ‘Cultivation’. Seasonal patterns in DOC, and CO₂ efflux reflected changes in soil water content, but MBC displayed no temporal response. Decreases in DOC and potential microbial respiration (RESP_mic) (i.e., microbially available C) also corresponded to or were preceded by increases in CO₂ efflux, suggesting that DOC provided C for microbial respiration. Despite similar MBC, DOC, RESP_mic, annual CO₂ efflux and aboveground C content between the two cover crops, greater aboveground net primary productivity and SOM in ‘Trios’ indicated that ‘Trios’ provided more soil C than ‘Rye’.

Introduction

Cover crops provide many services to agroecosystems. In particular, they decrease erosion, improve infiltration and reduce runoff, improve soil nutrient retention, and build soil organic matter (SOM) (Battany and Grismer, 2000, Jackson, 2000). Cultivation, a common practice in agroecosystems, has been linked to reductions in SOM, which occurs by oxidation of SOM protected within soil aggregates prior to tillage, and causes short-term perturbations in soil microbial biomass and activity (Six et al., 1999, Calderón et al., 2000, Calderón et al., 2001). No-till and minimum tillage practices have become increasingly popular as a means to reduce SOM loss. Much of the research on the influence of cover cropping, no-till, and reduced tillage practices on SOM has occurred in temperate regions (e.g., Six et al., 1999, Grandy and Robertson, 2007, Hermle et al., 2008), but recent findings suggest that combining cover crops with no-till practices and shifting tillage intensity may similarly enhance ephemeral and longer-term pools of SOM in Mediterranean and semiarid annual agroecosystems (Andrews et al., 2002, Hulugalle et al., 2006, Veenstra et al., 2007, Álvaro-Fuentes et al., 2008).

Mediterranean climates are characterized by cool, moist winters, which involve frequent wet–dry cycles, and dry, warm summers. In Mediterranean ecosystems, soil carbon dioxide (CO₂) efflux and microbial biomass carbon (MBC) have distinct seasonal dynamics, and in particular, portray strong increases after simulated rainfall (Lundquist et al., 1999a, Lundquist et al., 1999b, Fierer and Schimel, 2002, Steenwerth et al., 2005). These studies have occurred in many Mediterranean ecosystems, including cole crops, tomatoes, annual and perennial grasslands, and oak woodlands (Lundquist et al., 1999a, Lundquist et al., 1999b, Casals et al., 2000, Fierer and Schimel, 2002, Rey et al., 2002, Maestre and Cortina, 2003, Steenwerth et al., 2005). However, little information on the impacts of cover cropping and cultivation on soil respiration (i.e., root + microbial respiration), MBC, dissolved organic carbon (DOC), and other labile C pools presently exist for perennial agroecosystems (Carlisle et al., 2006). Perennial agroecosystems may support different soil carbon (C) dynamics than annual agroecosystems due to the lower frequency of soil disturbance by tillage.

Vineyards represent an ideal perennial agroecosystem in which to utilize cover crops and no-till practices to enhance SOM content and soil microbiological function. Cover crops grown in the alleys between grapevines are planted after minor soil preparation, with physical soil disturbance occurring as little as once per year. This is comparatively less frequent than in annual agroecosystems, which annually can support two to four crop rotations and experience multiple tillage passes. In the U.S. alone, grape-bearing acreage covered approximately 1.4 million ha in 2005, and vineyards now exist in every state, although 77% of all vineyard land use is in California (USDA/NASS). In California, the most recent documented estimates indicate that only approximately 16% of vineyards supported cover crops (Ingels and Klonsky, 1998). Vineyard cover crops are typically planted in the fall at the onset of precipitation (ca. October to November), receive no irrigation, and grow throughout the rainy season into late spring (ca. March to April) while the grapevines are dormant. Typically, they are mowed or tilled near spring budbreak to decrease potential frost damage to the grapevines. Given the benefits of cover cropping in other agroecosystems and the large spatial extent of vineyards, demonstrating potential benefits of cover cropping on soil C pools and microbiological function in vineyards has high potential for practical impact and adoption.

In order to understand how cover crops and cultivation affect soil C dynamics in a vineyard, we established our study in a Chardonnay vineyard in the Central Coast (Monterey Co., CA), a region with one of the largest contiguous stretches of vineyards in the world. The cover crop and cultivation treatments in the vineyard floor had been established 4 years previously as part of another study (Baumgartner et al., 2005). The cover crops, Trios 102 (Triticale × Triosecale) and Merced Rye (Secale cereale), had been selected due to their contrasting aboveground growth patterns. Typically, Trios 102 has less aboveground biomass and a more prostrate growth form relative to Merced Rye (S. cereale) early in the growth season. Trios 102 bolts later than Merced Rye, but they are nearly indistinguishable in morphology and aboveground by the end of the growth season in late spring just prior to mowing or tilling. In the present study, we (1) compared effects of cover crops and cultivation on soil C dynamics, CO₂ efflux, soil microbiological activity, and SOM content, (2) evaluated seasonal effects of soil temperature, water content, and precipitation on soil C dynamics, and (3) determined potential drivers of CO₂ efflux in this vineyard agroecosystem.