Changes in soil organic C, microbial C and aggregate stability under continuous maize and cereal cropping, and after restoration to pasture in soils from the Manawatu region, New Zealand

Abstract

Soil compaction can be a major limitation to continuous maize production on the heavier-textured soils of the Manawatu region, New Zealand. A survey was undertaken of commercially-operated farms that were producing continuous maize using conventional mouldboard ploughing and cultivation methods. The effects of cultivation on the proportions of water-stable macroaggregates, total organic C content, microbial biomass C and soil respiration were measured in the predominant soil type, a poorly-drained Kairanga silty clay loam (Typic Haplaquept). Three other regional soil types, a Manawatu silt loam (well drained), Kairanga silt loam (poorly drained) and Moutoa humic clay (very poorly drained) were incluced for comparison. The effects of cropping with wheat or barley, and the recovery of the soils after restoration to pasture, were also measured. Continuous cultivation with maize on the Kairanga silty clay loam for up to 11 years decreased the total C content in the top 20 cm of soil by 21%, microbial C by 49% and water-stable aggregates (greater than 2 mm) by 54% compared with the levels under long-term permanent pasture. Losses on cultivation of pastures were greater on Manawatu silt loam with decreases of 49% in organic C, 60% in microbial C and 98% of the greater than 2 mm aggregates. Changes were much less pronounced on the Moutoa soil with a greater initial organic matter content, and the soil rapidly established new, only slightly lower, equilibria. Cultivation for barley or wheat had a less detrimental effect than cultivation for maize on the total organic and microbial C contents and aggregate stability of Kairanga silty clay loam. There was a strong linear relationship between the decline in the proportion of stable aggregates and the loss of microbial C and organic C in the top 10 cm of cultivated Kairanga soil. The relationship was stronger between microbial C and aggregate stability ( R 2 = 77.8%) than total organic C and aggregate stability ( R 2 = 64%), but was only valid when soil organic C was declining, and was not significant in the soil at the 10–20 cm depth. Re-establishment of pasture caused a more rapid recovery in microbial biomass C than total C, and increased the proportion of organic C comprised of microbial C. However, recovery of the total organic and microbial C pools and aggregate stability was very variable, and after 4 years of pasture none of the sites had re-established the levels found under permanent pasture. The previous cropping history of these soils before being returned to pasture, rather than the organic or microbial C content, appeared to be of greater importance in controlling the aggregate stability characteristics.