Impact of Management on Soil Carbon and Nutrient Cycling and Storage Under Contrasting Farming Systems

Abstract

<p>Soil organic matter (SOM) is a key indicator of soil quality, regulating major soil processes and functions such as soil organic carbon (SOC) storage and cycling, microbial biomass and activity, and nutrient storage and cycling in agro-ecosystems. There has been increasing interest in these functions of SOM, and how they are impacted by management practices in different farming systems. However, there is limited understanding of how, and to what extent, soil structural units (termed "aggregate-size classes") with different SOM bioavailability, influenced by contrasting tillage practices, mediate these soil processes. Moreover, knowledge on the allocation dynamics of newly assimilated C and N in crop–soil systems, with implications for nutrient use efficiency and crop productivity, is also limited. An improved understanding of these inter-relationships will provide insights into identifying land management practices with potential to increase SOM storage, while enhancing plant available nutrients, nutrient use efficiency and therefore crop productivity at farm scale.</p><p> A series of experiments was carried out to enhance understanding of the impact of different management practices on key soil processes and functions such as: soil structural stability; carbon and nutrient cycling, availability and storage; microbial biomass and activity; and the coupling between plant C input and soil nutrient availability or N uptake by plants. These processes were examined in bulk soil and/or in different aggregate-size classes. The experiments reported in <strong>Chapters 2-5</strong> were performed using soils from three long-term (16-46 years) sites, <em>i.e</em>. the Condobolin (NSW) and Merredin (WA) sites, on a Luvisol, with a semi-arid or a Mediterranean climate, respectively, and the Hermitage (QLD) site, on a Vertisol, with a sub-tropical climate. The practices at Condobolin comprised conventional (CT) and reduced tillage (RT) with mixed crop-pasture rotation, no-till (NT) with continuous cropping, and perennial pasture (PP. The practices at Merredin comprised stubble either retained (SR) or burnt (SB) under direct-drilled continuous cropping. The practices at Hermitage comprised a factorial combination of CT, NT, SR, SB, with either 0 (0N) or 90 kg urea-N ha^-1 (90N) in a continuous cropping system. For the study in <strong>Chapter 2</strong>, dry and wet sieving techniques were used to separate mega- (> 2 mm), macro- (2-0.25 mm), micro-aggregate (0.25-0.053 mm) and silt-plus-clay (< 0.053 mm) fractions. The impact of management on soil structural stability, and total SOC and nutrient [N, phosphorus (P) and sulphur (S)] stocks in these differently sized aggregate classes was quantified at different soil depths (<em>i.e</em>. 0-10 cm, 10-20 cm and 20-30 cm depths). To understand the processes of SOC mineralisation and the release of plant available nutrients, as impacted by different tillage management practices, the surface layer (0-10 cm) bulk soil and soil aggregates [mega-aggregate, macro-aggregate and micro-aggregate (<0.25 mm)] were incubated at 22 ± 0.5 ̊C and 60% of water holding capacity for 126 days with or without crop residue amendments (<strong>Chapter 3-5</strong>).</p><p> The first study (<strong>Chapter 2</strong>) demonstrated that management practices with relatively low or no soil disturbance improved soil structure in the Luvisol at Condobolin, while the high clay content in the Vertisol at Hermitage may have overridden the effect of tillage, and therefore, management had minimal impact on soil structure stability. Further, this study found minor to modest impacts of long-term management practices on soil C gain and N, S and P stocks across the field sites. The so-called improved management practices such as perennial pasture at Condobolin, and no-till, stubble retention and fertilisation at Hermitage showed relatively higher SOC and nutrient stocks than management practices with disturbance through tillage plus stubble burning. The findings suggest that the least disturbed systems along with stubble retention and fertilisation can enhance agricultural sustainability by increasing SOC and nutrient concentrations, particularly in micro-aggregates or micro-structures.</p><p> The second study, (<strong>Chapter 3</strong>), showed that SOM has a significant fertiliser value in terms of the supply of plant available N, P and S, and management practices can significantly influence the release of plant available nutrients from SOM. In this experiment, bulk soils (Luvisol and Vertisol) collected from 14 long-term management practices across the three long-term field sites were incubated for 126 days. The mineralisation of SOC and the release of nutrients were higher in the CT <em>versus</em> RT and NT, and the SR <em>versus</em> SB practices in both soils. This study found a continuous release of plant available N across all the management practices over the study period, whereas, the release of available P and S was evident only during the first 30 days, after which P and S availability decreased, probably because microbial immobilisation or clay fixation of P and S predominated, particularly in the Vertisol. These findings suggest that SOM is a direct source of nutrients for crop growth, and management practices involving soil disturbance along with organic matter (residues) input can promote SOM mineralisation and the release of plant available nutrients in farming systems.</p><p> The third study, another incubation study (<strong>Chapter 4</strong>), suggested that SOM can continuously release plant available nutrients (particularly P and S) over 126 days after incorporation of residues. In this experiment, crop residues [canola (<em>Brassica napus</em>: δ¹³C 124‰) or wheat (<em>Triticum aestivum</em>: δ¹³C 461‰) stem] were added to Luvisol (δ¹³C -24.7‰) and Vertisol (δ¹³C -18.5‰) sampled from the contrasting tillage (CT or RT and NT) treatments and incubated for 126 days. This study found that crop residue input into the tilled systems stimulated native SOC mineralisation by ~100-300% across both soils. Both SOC mineralisation and the release of plant available nutrients varied with tillage intensity (CT or RT > NT), residue type (canola > wheat), and soil type (Vertisol > Luvisol). This study also found that crop residue input (<em>cf.</em> control) did not change the magnitude of net available N over the study period, possibly due to stronger N immobilisation than mineralisation. However, a significant amount of available P and S was released in both soils over 126 days. Therefore, this study suggests that, in addition to the likely release of available P and S from the residues <em>via</em> mineralisation, considerable quantities of available P and S may have been released from the soil reserves <em>via</em> positive priming of SOM mineralisation (as demonstrated in our study) and possibly <em>via</em> dissolution/desorption reactions in the soils.</p><p> Results from the fourth study (<strong>Chapter 5</strong>) demonstrated that the differently sized aggregate classes had a smaller effect, compared with the effects of tillage intensity, residue type and soil clay content/type, on the priming of native SOC mineralisation and nutrient (N, P and S) release dynamics in the soils. In this laboratory incubation study, the ¹³C-labelled canola or wheat stem residues were added into the three dry aggregate-size classes, collected from contrasting tillage systems on the Luvisol and Vertisol, and incubated for 126 days. This study found that crop residue input (<em>versus</em> no residue input) stimulated SOC mineralisation in all three aggregatesize classes in both soils. The native SOC mineralisation varied with tillage intensity (CT > RT > NT) (in the Luvisol only), residue type (canola > wheat), and aggregate-size classes (macro- ≥ micro- > mega-aggregates) in both soils. Crop residue input into the soil aggregates (<em>cf.</em> no residue input) maintained the release of available N over the study period, likely due to the dominance of microbial N immobilisation <em>versus</em> mineralisation induced by the relatively C-rich and nutrient-poor crop residues. An interesting finding is that incorporation of crop residues released a considerable amount of plant available nutrients (particularly P and S) from the soil aggregate reserves, most likely via different biological (e.g. priming) and chemical (e.g. nutrient desorption and mineral dissolution) mechanisms. Further, micro- and/or macro- <em>versus</em> mega-aggregates, canola <em>versus</em> wheat residue input, and Vertisol <em>versus</em> Luvisol had higher plant available nutrients in till <em>versus</em> no-till systems.</p><p> To understand the allocation dynamics of newly assimilated C and N in a canola crop-soil system with different tillage and N fertilisation treatments, a field-based ¹³C¹⁵N isotopic study was performed at Wagga Wagga, NSW (<strong>Chapter 6</strong>). Results from this study showed that short-term tillage and N fertilisation can increase belowground allocation of newly assimilated C and plant uptake of soil-released N, under a semi-arid environment. In this study, <em>in situ</em> ¹³CO₂ and urea-¹⁵N pulse labelling was conducted during the canola flowering stage. Despite no short-term effect of management practices on total SOC and N stocks, aggregate stability, microbial biomass, and ¹³C retention in different aggregate-size classes, this study found greater new root C input to 1 m depth, plant N uptake and canola seed yield under a low-intensity tillage (<em>cf.</em>. no-till) and N fertilisation (<em>cf.</em> no N) system.</p><p> To conclude, the studies showed that tillage along with stubble retention and/or N fertilisation can stimulate SOM decomposition and the release of plant available nutrients, while enhancing plant derived-C input and nutrient uptake, with positive implications for crop productivity in farming systems. On the contrary, the least disturbed systems such as perennial pasture or no-till along with stubble retention and N fertilisation can increase SOC and nutrient concentrations in micro-structures, although these systems (<em>cf.</em>. tilled systems) gave modest gains in SOC and nutrient stocks. Using tillage to encourage nutrient release should be planned strategically, as the timing of nutrient release from the stimulated SOM decomposition needs to be aligned with crop demand to maximise efficiency of use of nutrients. A further consideration is the trade-off with climate change mitigation, as encouraging SOM mineralisation <em>via</em> tillage will decrease SOC stocks, particularly in surface soil layers.</p>