Executive Summary
This investigation compared
the effects of stubble burning and stubble retention on a range of soil
chemical and biological properties from paired sites on eight properties,
four pairs, with different histories and soil types in the Murrumbidgee
Irrigation Area. The chemical properties included total soil carbon, nitrogen,
phosphorus and sulphur (C, N, P and S). Soil biological properties included
the size and activity of the microbial biomass, the microbial and metabolic
quotients, the rate of cellulose decomposition and a range of information
concerning the composition of the microbial population derived from fatty
acid methyl ester (FAME) analysis.
Each of the soils from the
sites retaining stubble had significantly more C, N, P and S than the corresponding
property that burnt stubble despite differences in other management strategies.
The biological properties of the stubble retained soils suggest that they
would be considered healthier than the corresponding stubble burnt soils
and two measurements used to indicate soil stress, the microbial quotient
and a stress indicator derived from FAME analysis, suggest that the stubble
retained soils were less stressed than the stubble burnt soils. The findings
also support the view that for rice, or other summer crop stubble, to decompose
at the fastest rate in the field it should be incorporated as soon as possible
after harvest, before winter, and not during the sowing of the following
summer crop, before summer.
The similarity of the C:N:P:S
ratios at all the sites, regardless of stubble treatment, suggest that
the soil organic matter at all the sites could be similar in composition
regardless of the type of organic matter inputs, further work is needed
to clarify this. Further, the similarity of the ratios to the published
ratios for soil humic material suggest that the majority of the organic
matter at these sites is humic type material with little fresh organic
matter remaining. Although it is well known that adding nitrogen fertilizer
can overcome nitrogen tie-up and enhance stubble decomposition the precise
values of these ratios suggest that phosphorus and sulphur tie-up could
also be potential problems and that adding phosphorus and sulphur could,
under certain circumstances, also enhance stubble decomposition. It is
suggested that there needs to be a balance of available nutrients to optimise
nutrient availability and potential carbon sequestration in stubble retained
systems, especially in the early years after switching from a stubble burning
regime.
Last updated: February
2006 Copyright RIRDC
http://www.rirdc.gov.au/reports/RIC/05-195sum.html
