Biological and other soil parameters response to winter cover crops in a mediterranean irrigated system

ABSTRACT Replacing the traditional fall-winter fallow with cover crop (CC) contributes to benefit both soil and the subsequent plant systems. The study of soil microbial activity is an important aspect to understand soil-plant interactions and to monitor the effect produced by changes of management practice in a short term. The research was carried out in two areas with different climates and soil characteristics. The first area was in Spain under Mediterranean conditions and the second in The Netherlands with a temperate oceanic climate. The main study for this work was carried out in a long-term field experiment in Aranjuez (Madrid) with a Mediterranean semiarid climate. The trial was established in 2006 with minimum tillage in an area with an alkaline soil and symptoms of soil degradation due to conventional tillage and maize monoculture, which are common practices in the area. The winter treatments were: barley (Hordeum vulgare L.), vetch (Vicia villosa L. or V. sativa L.) and fallow. During the summer, the main crops were maize (Zea mays L.) or sunflower (Helianthus annuus L.). The biological soil quality indicators were evaluated on the one hand through a selection of soil biological parameters over the years, and on the other hand through a detailed study of the arbuscular mycorrhizal fungi (AMF) in the soil and plants. In the first stage, different biological indicators were selected to evaluate the CC effect during the long-term field experiment (10 years). Enzymatic activities, microbial biomass C, both easily extractable and total glomalin fractions were evaluated in the soil along with soil respiration and also the total organic C content. In the second stage, the CC effect on the AMF activity in the subsequent main crop was studied in variables directly related to AMF: mycorrhizal colonization, length of hyphae and AMF spores. In addition, there were other two variables indirectly related to AMF: the easily extractable glomalin-related soil protein and the β-glucosaminidase activity. Nutritional parameters in the soil (available P) and plant (N, P and C concentrations) and maize height were obtained during the main crop, and yield at harvest. Those parameters were measured to monitor the positive CC effect in several sampling dates during the main crop growth. Cover crop species affect the mycorrhizal activity in different ways in the subsequent main crop. Legumes and grasses are common AMF hosts, but there are other species that cannot form mycorrhizal symbiosis, such as crucifers. Mycorrhizal colonization might vary depending on the different CC species used, climatic and soil conditions and also on the main subsequent crop. Therefore, the legacy of different CC on the AMF colonization in two subsequent crops was studied in a second field experiment under temperate oceanic climate. The winter treatments were: fallow, perennial ryegrass (Lolium perenne L.), white clover (Trifolium repens L.), vetch (V. sativa L.) and radish (Raphanus sativus L.), and the mixtures ryegrass+clover and radish+vetch. The mycorrhizal colonization was studied in the roots of two main crops: oat (Avena sativa L.) and endive (Cichorium endivia L.). In the long-term experiment, the biological parameters and total organic C content were stimulated by the CC treatments. Both easily extractable and total glomalin fractions were the parameters earlier enhanced by CC (in 2008). Then, β-glucosaminidase and dehydrogenase activities and soil respiration showed improvements in 2010. Microbial biomass C did not show earlier increments than total organic C, both parameters were improved by both CC in 2012. Most of the parameters were higher in the first 5 cm depth compared with the second layer (5-20 cm). Overall, after CC and minimum tillage establishment in 2006, the biological parameters related to soil quality increased through the long-term experiment under Mediterranean conditions. The response of these parameters was different and the effect was more notable in the upper layer. The mycorrhizal activity was affected by the CC used in the Mediterranean long-term experiment. In the two maize campaigns (2012-2013 y 2013-2014), barley improved the mycorrhizal colonization in maize by 19%, the length hyphae 85% and the AMF spores 64% compared with fallow. The vetch treatment, on the contrary, did not enhance most of the AMF parameters. The easily extractable glomalin fraction was correlated with the variables directly related to AMF, which contributed to include it as a variable directly related to AMF. Both CC contributed to improve the aggregate stability, due to increments in the length hyphae and glomalin content. The AMF stimulation by the barley CC enhanced shoot P concentration in the maize crop during the first growth stages. The vetch treatment had the highest shoot N concentration in maize plants and also β-glucosaminidase and phosphatase activities. The study of AMF parameters during various sampling dates showed that the mycorrhizal colonization was the parameter more sensitive to time. The differences on the other AMF parameters were constant over the time. In the study under temperate oceanic climate, similar results were obtained compared with the Mediterranean experiment. Differences on mycorrhizal colonization between grasses, legumes and fallow were similar in both trials. That means that the grass stimulated the micorrhization in both main crops while the legumes showed no differences with fallow. Overall, ryegrass and ryegrass+clover treatments had the highest colonization compared with the other treatments (increments of 60% in oat and 188% in endive compared with fallow). The mixture ryegrass+clover showed no improvements in the mycorrhizal colonization compared with ryegrass monoculture, despite being a grass-legume combination. In the oat roots, the radish CC negatively affected the mycorrhizal colonization, decreasing by 51% compared with fallow. The mixture radish+vetch did not counterbalance the negative effect of radish (49% lower than fallow). The replacement of fallow by CC together with tillage reduction constitutes good agricultural practice that can mitigate and even reverse negative effect derived from the intensification of cropping systems. This study demonstrates that part of the benefits provided by CC in soil quality and plant growth is through biological parameters enhancement and particularly via AMF stimulation.