Effect of alternative cropping management on soil organic carbon

Abstract

Concerns about declining soil organic carbon (SOC) and increased greenhouse gas emissions due to practices such as intensive tillage and bare fallows have encouraged the adoption of practices such as no-tillage, crop rotations and residue retention. However, whilst no-till farming is suited for broadacre crops, it has not been widely adapted for most vegetable production systems. Vegetable production systems, especially organic ones, routinely use tillage to prepare beds and manage weeds. These tillage operations break soil structure and aggregates, which is known to accelerate losses of SOC stocks. Despite requiring multiple tillage operations, the vegetable systems are also characterised by little or no crop residue input, potentially further reducing SOC stocks. The effect of sweet corn ('Zea mays L. var. rugosa') residue management (RM; i.e. incorporation or removal) in a corn-cabbage ('Brassica oleracea' L.) rotation on SOC parameters in two soil management systems (SMS; i.e. organic and conventional) was examined because crop residue incorporation and application of organic fertilisers could be ways to counteract loss of SOC due to tillage in vegetable systems. The principal aim of this thesis was to examine the effect of RM in the two SMS on soil total carbon (TOC) concentrations and stock, soil carbon fractions and microbial biomass carbon (MBC) through a field experiment of a corn/cabbage rotation over two years. A laboratory experiment was performed to separate the confounding factors of SMS in the field experiment, i.e. herbicide and mineral fertilisers in the conventional SMS, and cultivation and organic fertilisers in the organic SMS. To supplement the field experiment, another laboratory experiment focused on how two potentially opposing determinants of TOC, residue incorporation and simulated tillage (sieving), influence the emission of CO₂-C. Although, the research objectives of this thesis are focused on SOC, agronomic and fertility parameters, the essential components of a crop production system, were also considered.