Abstract
Carbon (C) and nitrogen (N) allocation and assimilation are coupled processes, likely influencing C accumulation, N use efficiency and plant productivity in agro-ecosystems. However, dynamics and responses of these processes to management practices in semi-arid agro-ecosystems are poorly understood. A field-based 13CO2 and urea-15N pulse labelling experiment was conducted to track how C and N allocation and assimilation during canola growth from flowering to maturity were affected by short-term (2-year) tillage (T) and no-till (NT) with or without 100 kg urea-N ha−1 (T-0, T-100, NT-0, NT-100) on a Luvisol in an Australian semi-arid region. The T-100 caused greater (P < 0.05) belowground C allocation and higher (P < 0.05) translocation of soil N to shoots and seeds, compared to other treatments. Microbial N uptake was rapid and greatest in the fertilized (cf. non-fertilized) treatments, followed by a rapid release of microbial immobilized N, thus increasing N availability for plant uptake. In contrast, management practices had insignificant impact on soil C and N stocks, aggregate stability, microbial biomass, and 13C retention in aggregate-size fractions. In conclusion, tillage and N fertilization increased belowground C allocation and crop N uptake and yield, possibly via enhancing root–microbial interactions, with minimal impact on soil properties.
Highlights
Plants allocate recently photo-assimilated carbon (C) to aboveground and belowground organs to support their structural and non-structural components and metabolic processes[1, 2], influencing the C source sink balance[3] and nutrient cycling in terrestrial ecosystems[4]
Less attention has been given to the understanding of how different management practices influence whole-plant C and N allocation, assimilation and interactions under field conditions[7], in semi-arid dryland agro-ecosystems, where 50% or more of plant available N is derived from soil organic matter (SOM) mineralisation[16]
We found that tillage intensity and N fertilization influenced belowground C allocation, root-microbial interactions and plant N uptake, possibly driven by changes in C source–sink relations among the aboveground and belowground pools[3]
Summary
Plants allocate recently photo-assimilated carbon (C) to aboveground and belowground organs to support their structural and non-structural components and metabolic processes[1, 2], influencing the C source sink balance[3] and nutrient cycling in terrestrial ecosystems[4]. In a recent field study, An et al.[10] reported 12–15% allocation of the newly assimilated 13C to belowground pools (such as soil, roots and microbial biomass) 15 days after pulse labelling in differently managed maize–soil systems. The processes of C and N accumulation in soil aggregates may be related to the extent of belowground C allocation, which may concurrently enhance microbial activity, N cycling and soil aggregate stability[7, 26, 27] As these soil processes may be impacted by tillage and N fertilization[15, 28], a better understanding of the allocation, fate and retention of C and N in soil aggregates of different size classes is needed to acquire insights into pathways of SOM accumulation under contrastingly managed cropping systems. We hypothesized that (i) tillage with N fertilization could increase belowground C allocation through enhancement of root growth, stimulating microbial activity and interactions, compared to tillage without N fertilization and no-till with and without N, leading to the greatest plant N uptake from soil; (ii) uptake of soil-released N by microbes could be more prominent under tillage and N fertilization than under no-till (with and without N) and tillage without N; and (iii) tillage (vs. no-till) would have minimal impact on aggregate stability and C and N storage in stable aggregates
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