Visual Assessment of Soil Structure as an Early Indicator of Soil Quality in Response to Intensive Rotational Grazing

Grasslands can play a crucial role in mitigation of global warming by serving as carbon sink. Nevertheless, to achieve the grasslands’ potential, sustainable management is of the utmost importance as it determines system’s productivity and ecosystem services. Due to the increasing demand for animal products in developing countries, grazed areas increase exponentially in the tropics, mainly due to unsustainable management leading to low productivity and soil degradation. We evaluated the impact of intensive rotational grazing management (IRG) on early indicators of soil quality following land-use change based on on-farm observations of visual soil characteristics using two different widely used assessment methods: visual soil assessment-VSA and visual evaluation of soil structure-VESS. Correlation of visual methods were combined with measurements of soil macrofauna abundance and physical properties (e.g. bulk density, soil porosity). The IRG established in two study sites in Colombia was compared with traditional long-term continuous grazing with low stocking rate (1 LU ha-1). The IRG was based on rapid (1 day) cattle grazing in paddocks with high stocking rate (180 LU ha-1) followed by 60 days of recovery. In both study sites, IRG increased considerably total stocking rate to 4 LU ha-1 while improving grassland composition by enabling more valuable species, which contributed to soil quality and increased grassland productivity. Both VSA and VESS discriminated IRG-managed sites in less than one year after IRG adoption. Our results demonstrate that visual soil assessment is a useful mean for evaluation of soil quality and grassland productivity. Furthermore, VSA and VESS seemed to be more suitable in discriminating among management in early stages, when compared to commonly used soil physical properties, and were strongly correlated mainly to the abundance of earthworms. Furthermore, our study confirms the importance of grazing management in soil quality and ecosystem productivity/sustainability

Liming reduces N2O emissions from Mediterranean soil after-rewetting and affects the size, structure and transcription of microbial communities

In Mediterranean regions, the accumulation of nitrogenous substrates in soil during summer fallow period has been linked to pulses of N2O emissions upon soil rewetting. Although the mechanisms of N2O emission after soil rewetting have been previously studied, potential mitigation of agronomic practices on N2O pulses is still poorly understood. We studied the N2O emissions after rewetting of degraded soils managed by no-tillage (NT) and conventional tillage (CT), both with or without lime application under laboratory conditions. The soil was rewetted to 50 and 100% of field capacity (FC) and the N2O fluxes, size, structure and gene transcription of the microbial communities involved in the N2O emissions were evaluated. Liming reduced the cumulative N2O emission by more than 70 and 65% respect to the unamended soil after soil rewetting to 50% and 100% of FC, respectively, whereas no significant effect of tillage on N2O emission was observed. Liming strongly influenced the size and structure of ammonia oxidizing bacteria (AOB) and denitrifier communities, increased the transcription of the nirK after soil rewetting, while transcription of genes encoding nitrification enzymes was undetectable. Tillage slightly affected the nitrifier and denitrifier communities, with CT increasing the size of nosZ community. The results indicated that the N2O pulse after soil rewetting was caused by denitrification rather than nitrification. In addition, the increase of nirK transcription suggested that the mitigation of N2O emissions observed in limed soils was due to changes in the denitrification process, possibly with a higher or more efficient reduction of N2O to N2. This study showed the potentials of NT and liming management to mitigate the N2O emission from degraded Mediterranean soils after soil rewetting due to changes in the efficiency of the denitrification process. Our results also showed the usefulness of coupling determinations of gas emission, microbial community structure and gene transcription to clarify the underlying mechanisms of N2O emissions from soil