Short-term impact of crop diversification on soil carbon fluxes and balance in rainfed and irrigated woody cropping systems under semiarid Mediterranean conditions

Purpose Diversification practices such as intercropping in woody cropping systems have recently been proposed as a promising management strategy for addressing problems related to soil degradation, climate change mitigation and food security. In this study, we assess the impact of several diversification practices in different management regimes on the main carbon fluxes regulating the soil carbon balance under semiarid Mediterranean conditions. Methods The study was conducted in two nearby cropping systems: (i) a low input rainfed almond (Prunus dulcis Mill.) orchard cultivated on terraces and (ii) a levelled intensively irrigated mandarin (Citrus reticulata Blanco) orchard with a street-ridge morphology. The almond trees were intercropped with Capparis spinosa or with Thymus hyemalis While the mandarin trees were intercropped with a mixture of barley and vetch followed by fava bean. Changes caused by crop diversifications on C inputs into the soil and C outputs from the soil were estimated. Results Crop diversification did not affect soil organic carbon stocks but did affect the carbon inputs and outputs regulating the soil carbon balance of above Mediterranean agroecosystems. Crop diversification with perennials in the low-input rainfed woody crop system significantly improved the annual soil C balance in the short-term. However, crop diversification with annual species in the intensively managed woody crop system had not effect on the annual soil C balance. Conclusions Our results highlight the potential of intercropping with perennials in rainfed woody crop systems for climate change mitigation through soil carbon sequestration.

Estimation of daily soil CO2 flux using a single-time-point measurement

Information on soil CO2 emissions can be used in conjunction with data for C inputs from plants to estimate the soil C balance. Many studies assume a single-time-point measurement of soil CO2 flux taken in a day (Fh) is equal to its daily average value (Fd), which could result in over- or underestimation. A model using Fh, the temperature at the time the Fh is measured (Th), daily average temperature (Td) and Q10 factor to predict Fd was tested with extensive measurements of soil CO2 flux, temperature and moisture over 60 d from various treatments (no-till wheat, summer fallow and stubble, etc.) on a Swinton silt loam near Swift Current, Saskatchewan, Canada. A distinct hysteresis between flux and soil moisture was observed following rain events. Therefore, data for five rainy days were excluded from the analysis as the model does not consider the effect of hysteresis. Calculated Q10 factors were 1.86 and 1.54 for soil (Ts) and air (Ta) temperatures, respectively. The model using either Ts or Ta improved prediction of Fd in both calibration (49 d) and validation (6 d) datasets compared with Fh. Values of Fh measured in 6 yr were higher than modelled values of Fd in 96% of the 1602 treatment-days, hence if Fh is assumed to be equal to Fd, the averaged overestimation would be 17%.Key words: Soil CO2 flux, Q10 factor, temperature

Soil carbon stabilization along climate and stand productivity gradients in black spruce forests of interior Alaska

The amount of soil organic carbon (SOC) in stable, slow-turnover pools is likely to change in response to climate warming because processes mediating soil C balance (net primary production and decomposition) vary with environmental conditions. This is important to consider in boreal forests, which constitute one of the world's largest stocks of SOC. We investigated changes in soil C stabilization along four replicate gradients of black spruce productivity and soil temperature in interior Alaska to develop empirical relationships between SOC and stand and physiographic features. Total SOC harbored in mineral soil horizons decreased by 4.4 g C·m–2 for every degree-day increase in heat sum within the organic soil across all sites. Furthermore, the proportion of relatively labile light-fraction (density <1.6 g·cm–3) soil organic matter decreased significantly with increased stand productivity and soil temperature. Mean residence times of SOC (as determined by Δ14C) in dense-fraction (>1.6 g·cm–3) mineral soil ranged from 282 to 672 years. The oldest SOC occurred in the coolest sites, which also harbored the most C and had the lowest rates of stand production. These results suggest that temperature sensitivities of organic matter within discrete soil pools, and not just total soil C stocks, need to be examined to project the effects of changing climate and primary production on soil C balance.