Six years of different tillage systems affected aggregate-associated SOM in a semi-arid loam soil from Central Iran

Abstract

Tillage practices potentially influence soil organic matter (SOM) storage and dynamics, which is important for sustainable cropping practices and agricultural productivity, and carbon (C) sequestration in soil. The objective of the current study was to quantify changes in bulk SOM and aggregate-associated SOM in a loam soil following four tillage practices for a period of six years (2005–2011). The amount of bulk soil organic C and nitrogen (N), aggregate stability, and C and N associated with both macroaggregates (4–0.250mm) and microaggregates (0.250–0.053mm) at the 0–20cm depth were evaluated for a semi-arid loam soil under moldboard (MP), disk (DP), chisel (CP) and rotary (RP) plows with a similar addition of organic residues to the soil under all the tillage systems. The experiment was designed as a randomized complete block with three replicates of each tillage treatment. Soil samples were obtained during the experiment in 2008, 2009 and 2011. SOM (C and N) pool did not vary between tillage treatments, while the pools of C and N averaged across tillage treatments were increased with time. This suggests that changes in SOM were more evident with addition of plant residues than with tillage systems. Soil aggregate stability indices (i.e., mean weight diameter and aggregate ratio values) were significantly greater in CP and RP than MP and DP practices in all the sampling years mainly due to an increase of the macroaggregate fraction, and concurrently a decline of the microaggregate fraction. Similarly, soil aggregate stability indices were increased with increasing the time period of experiment in all the tillage systems and when compared with the initial values before the experiment. CP and RP practices increased the pools of C (10–11%) and N (13–15%) associated with the macroaggregate-size fraction, while decreasing C (50–66%) and N (48–61%) associated with the microaggregate-size fraction. Furthermore, differences in SOM and aggregate stability between MP and DP practices and between CP and RP practices were not statistically significant. It is concluded that a reduction in tillage intensity under both CP and RP would not improve total SOM in the short-term, but would improve soil structure and shift SOM from the microaggregate to macroaggregate fractions without a substantial change in the whole SOM of the studied calcareous soil. Soil aggregate stability and SOM associated with aggregate fractions appear to be more sensitive to short-term tillage practices than the whole SOM in this semiarid soil.