Soil organic matter dynamics in mixed farming systems

Abstract

In mixed farming systems, lime is commonly incorporated using tillage. The subsequent aggregate breakup and mixing of carbon (C)-rich plant residues into the soil increases microbial mineralization of soil organic matter (SOM) leading to SOM losses and declines in agricultural productivity. Increases in SOM have occurred following incorporation of C-rich wheat residue when stoichiometric supplementary nutrients were applied to meet both microbial and crop requirements in annually cropped soils. However, this mechanism is untested in mixed farming systems. While the pasture phase of a mixed farming system builds SOM, there is substantial mineralization during the tilled transition to crop. It remains unclear how treatments which alter microbial dynamics affect this SOM mineralization. This thesis is presented as a series of conceptually linked papers that together address this knowledge gap. The aim was to identify management practices that reduce the substantial SOM loss in mixed farming systems by assessing short-term SOM dynamics during the transition to crop with changes in soil fertility and acidity intended to alter microbial dynamics. Soil C was measured during the transition from a long-term pasture to crop phase. The pasture was managed under experimental conditions with 20 years of phosphorus (P) fertilization (high fertility) compared to a non-P fertilized control (low fertility). This provided breadth in experimental results when assessing the soil C post-till. Treatments in the transition to crop investigated the impact of changes in microbial dynamics on soil C (control, lime, lime+nutrient). Two additional treatments with sugar cane mulch (lime, lime+nutrient) enabled isotope tracking of the C4 mulch breakdown, as the soil had previously grown C3 plants. These treatments assessed the impact of nutrient supply on the microbial transfer of the mulch into soil C. Pasture fertilization increased whole soil C (<5 mm) by 12 Mg C ha-1 to 110 cm depth and increased fine fraction C (<0.4 mm) to at least 60 cm depth. Following the transition to crop, fine fraction soil C loss was affected by the interaction of lime with nutrient supply, with higher loss in the limed low fertility soil (12.3 %) unless supplementary nutrients were added (7.0 %) (10 cm); whereas, lime incorporation reduced C loss in the high fertility soil (9.6 %). Nutrient addition reduced pre-existing soil C loss and increased sugar cane mulch breakdown with a decline of 47 and 38 mg 13 C kg-1 soil in the high fertility nutrient and low fertility non-nutrient treatments, respectively. Overall, pasture fertilization increased soil fertility leading to higher soil C post-till. Lime incorporation reduced soil C unless soils had high initial fertility or supplementary nutrients. Collectively, the substantial SOM loss in mixed farming systems can be reduced by balancing lime incorporation with either high initial soil fertility or stoichiometric supplementary nutrient addition. These new insights support a paradigm shift in thinking from providing nutrient and lime inputs that meet plant needs only, to providing inputs that meet the needs of the plant-soil system. This may reduce SOM loss and hence increase agricultural productivity, which is important for food security. Show more