Effect of no-tillage and liming practices on soil microbial parameters in degraded acid soils of sw spain

Resumen

Loss of soil organic carbon (SOC) and soil acidification as a result of unsustainable agronomic practices are two of the main concerns of the present agriculture at global scale, affecting food security and environmental quality. Recently, adoption of more environmental-friendly techniques, and conservation agriculture in particular, has been identified as a suitable practice for restoring the fertility of degraded soils. In the case of acid degrades soils, the combination of no-tillage (NT) with surface lime application has been shown to restore the SOC pools, improve soil physical properties and soil-water relationships, alleviate the acidity and Al3+ toxicity, and increase crop yields. Nevertheless, as several important aspects of the combined application of NT with surface liming has not been addressed by the previous research, this thesis aims at filling these knowledge gaps and at identifying the interactive effects of NT and liming on soil microbial activity and carbon (C) and nitrogen (N) cycling with a special emphasis on the seasonality of the Mediterranean climate. An exhaustive literature review prior to this thesis identified six gaps of knowledge, which were summarized in: (i) soil biological properties and their evolution during the year under Mediterranean climate, (ii) the impact of NT and liming on N transformation during the summer fallow period and (iii) the subsequent rewetting event, (iv) the combined effect of both practices on C and N mineralization and (v) soil protease activity and N availability, and (vi) on the plant symbiotic interactions with arbuscular mycorrhiza fungi (AMF) and nutrient uptake. To address these under-investigated topics, specific experiments were designed and performed in the long-term experimental field located in the Cañamero raña in Cáceres (SW Spain). The Cañamero experimental site/field was established in 2005 and NT, as an alternative to traditional tillage (TT), and liming were combined in a full factorial design. A mixture of oat (Avena sativa L.) and vetch (Vicia sativa L.) was cropped every season for forage under rainfed conditions. The first experiment evaluated the soil enzymatic activity and microbial biomass in four different sampling times during the same year (January, April, July, October) aiming to evaluate the interaction between the treatments and the Mediterranean seasonality. The results showed a general increase of soil microbial biomass and enzymatic activity in the topsoil of NT treatments which was related to the increase of SOC and the amelioration of the extreme drought during several water-limited months. However, liming slightly affected soil enzymatic activities, decreased the microbial biomass C and increased microbial biomass N, which suggests as a shift in the microbial communities. The amelioration of drought under NT probably caused the maintenance of higher microbial biomass and enzymatic activity during the summer period and, thus, increased soil microbial resilance to drought stress. A second experiment aimed at addressing the impact of NT and liming on the gross N transformation rates during the summer fallow period using 15N pool dilution technique. Liming increased both gross N mineralization and nitrification while traditional tillage increased gross N immobilization. Tillage did not affect gross nitrification but the effect of liming on gross N mineralization was enhanced in NT plots. During the summer fallow, mineralization, nitrification and microbial immobilization decreased and ammonium was accumulated, probably as a consequence of the decoupling between production and consumption rates of ammonium. The increase of gross N mineralization and nitrification caused by liming enhances the risk of N losses via nitrate leaching and gaseous losses after soil rewetting in autumn, especially when combined with NT. The third experiment focused on the evaluation of the effects of both agronomic practices on the N2O emissions after soil rewetting at the end of the summer fallow, and the interplay of soil microbes. Soil samples from the experimental field were rewetted in laboratory under controlled conditions and the soil-atmosphere N2O fluxes were determined periodically. Liming application reduced the cumulative N2O emission by more than 60% respect to the unamended soil after soil re-wetting. These findings show the usefulness of liming to mitigate the N2O emission after the summer rains in Mediterranean areas. Further, I found high impact of liming on the size, structure and transcription of selected microbial populations involved the N cycle and N2O emissions. The high transcription rate of nirK-bearing microbes and the absence of nitrifier transcription during the N2O peak suggest that the N2O was originated by denitrification rather than by nitrification. In addition, the strong stimulation of nirK transcription by liming indicates that the mechanism by which liming suppress the N2O emission was related with changes in the denitrification process. The fourth experiment aimed to understand the interactive effects on C and N cycling and their combined effect on microbial indicators using a Structural Equation Model (SEM). The SEM revealed that C mineralization was only affected by tillage practices while net N mineralization was increased by NT and liming amendment. The absence of relevant effects of amendment on C mineralization suggests a possible decoupling between the C and N mineralization processes with potential implications for N losses. In addition, SEM revealed that the impact of tillage practice on C and N mineralization was mediated by changes in the microbial biomass, which highlights the importance of the inclusions of microbial parameters in the nutrient cycling studies as affected by agronomic practices. The fifth experiment was designed to understand the effect of tillage practice and liming on depolymerization of organic proteinaceous compounds, a critical step of N cycling in agroecosystems. The activity of soil casein protease (an enzyme degrading high-molecular weight proteinaceous compounds), N-α-benzoyl-L-argininamide protease (BAA) (targeting low-molecular weight compounds) was determined in three soil layers. Higher casein protease and a lower BAA protease activity was observed in limed plots This imbalance in enzymatic activity, together with an observed reduced extractable organic C and microbial biomass C in limed plots suggests a reduced microbial immobilization of mineral N. Such situation may result in an accumulation of intermediate metabolic products (i.e. monomers and amino acids), therefore inducing reduced mineral N losses and/or changes in N acquisition preferences by plants. The sixth and last experiment analyzed the impact of NT and liming on plant AMF mycorrhization and the effects on plant growth and N and P contents during three growth stages of oat and vetch. The oat plant biomass and N content were increased by NT and liming while vetch biomass and N content was increased only by liming throughout the whole growing season. In addition, AMF root colonization of both crops was higher under NT and limed plots, despite of the relatively high content of available P in all treatments. The alleviation of soil acidity and enhancement of soil properties related with NT previously described, linked with the higher plant mycorrhization, suggests that, under Mediterranean conditions, the combination of these two common agronomic practices may increase both yields and nutrient use efficiency in acid soils. In conclusion, this thesis shows that the combination of NT and liming is a sustainable practice to restore acid degraded soils, enhancing the microbial activity and stimulating soil C and N cycling. In addition, the thesis revealed the impact of both agronomic practices in the summer fallow period characteristic of the Mediterranean climate which is critical for N losses. The obtained results show the usefulness of liming to reduce the N2O emission after the summer rains both higher N uptake by crops, which suggests a mitigation of the negative environmental effects of agriculture. This thesis may contribute to promote the adoption of more suitainable agronomic practices in acid soils under Mediterranean conditions as well as to identifying and understanding of their potential environmental impacts.