The organic carbon stock in biomass and soil profiles sampled from nearby paddocks with different land-use histories was estimated at 7 sites in the upper Liverpool Plains catchment and the Manilla district of north-western New South Wales, Australia. The distribution of soil carbon concentrations over a depth of 2 m was significantly affected by site and land use. Continuous cultivation and cropping over ≥20 years significantly depleted carbon concentrations compared with grassy woodlands in the surface 0.20 m at all sites and to a depth of 0.60 m at 3 sites. Depth of sampling (0–0.20 v. 0–1.0 m) significantly affected the differences between land uses at most sites regarding estimates of the stock of soil carbon. These results show that differences in soil carbon concentrations and stock size do not remain constant with depth between contrasting land uses. However, comparisons between land uses of the total amount of carbon stored were dominated by the number of trees per ha and the size of the trees in grassy woodlands. The implications of these results for carbon accounting are discussed.
Australian agriculture contributes an estimated 16% of all national greenhouse gas emissions, and considerable attention is now focused on management approaches that reduce net emissions. One area of potential is the modification of cropping practices to increase soil carbon storage. Here, we report short–medium term changes in soil carbon under zero tillage cropping systems and perennial vegetation, both in a replicated field experiment and on nearby farmers’ paddocks, on carbon-depleted Black Vertosols in the upper Liverpool Plains catchment. Soil organic carbon stocks (CS) remained unchanged under both zero tillage long fallow wheat–sorghum rotations and zero tillage continuous winter cereal in a replicated field experiment from 1994 to 2000. There was some evidence of accumulation of CS under intensive (>1 crop/year) zero tillage response cropping. There was significant accumulation of CS (~0.35 Mg/ha.year) under 3 types of perennial pasture, despite removal of aerial biomass with each harvest. Significant accumulation was detected in the 0–0.1, 0.1–0.2, and 0.2–0.4 m depth increments under lucerne and the top 2 increments under mixed pastures of lucerne and phalaris and of C3 and C4 perennial grasses. Average annual rainfall for the period of observations was 772 mm, greater than the 40-year average of 680 mm. A comparison of major attributes of cropping systems and perennial pastures showed no association between aerial biomass production and accumulation rates of CS but a positive correlation between the residence times of established plants and accumulation rates of CS. CS also remained unchanged (1998/2000–07) under zero tillage cropping on nearby farms, irrespective of paddock history before 1998/2000 (zero tillage cropping, traditional cropping, or ~10 years of sown perennial pasture). These results are consistent with previous work in Queensland and central western New South Wales suggesting that the climate (warm, semi-arid temperate, semi-arid subtropical) of much of the inland cropping country in eastern Australia is not conducive to accumulation of soil carbon under continuous cropping, although they do suggest that CS may accumulate under several years of healthy perennial pastures in rotation with zero tillage cropping.
We measured the volumetric water content (using neutron moisture meters) and height changes (using rods fixed in the soil at different depths) of several soil layers during swelling or shrinking of a heavy clay soil in the Liverpool Plains of New South Wales under 3 treatments: (1) a sorghum–fallow–lucerne, (2) continuous lucerne, and (3) continuous fallow. Treatment 1 resulted in weak drying and shrinking (in the sorghum phase), followed by weak wetting and swelling (fallow), and then strong drying (lucerne) accompanied by shrinkage of up to about 140 mm in the top 3 m of the profile. Treatment 2 resulted in repeated wetting and drying, with repeated swelling and shrinking. The continuous fallow treatment (3) had an initially dry profile (following a lucerne crop) and we roughened (chisel ploughed) the surface to promote infiltration. This resulted in a strong wetting event with swelling of up to about 200 mm in the top 3 m of the profile. There were clear trends between water storage change and height change for the first and third treatments, but not for the continuous lucerne treatment.The relationship between water storage change and height change was consistent with a constant of proportionality (α) of 0.33, unchanging with depth. The data were also consistent with α increasing slightly in deeper soil layers. There was no evidence of α approaching 1 (which is sometimes reported in the literature for wet soil) or zero (which is often reported in drier soil and equated with residual shrinkage), nor of differences in behaviour on wetting and drying.The height change measurements permitted estimation of the true water storage changes with swelling or shrinkage taken into account. Failure to take volume change into account (i.e. of treating the neutron moisture readings as correct) led to errors in the estimated profile water storage of up to 80 mm. The apparent (uncorrected for volume change) profile moisture storages were readily corrected using a simple analysis based on the assumption of proportionality between water storage change and volume change. The analysis leads to a correction 1/(1 – αθ) (where θ is the volumetric water content). For our data the correction was about 15–20%.
Deep drainage or drainage below the bottom of the profile usually occurs when rain infiltrates moist soil with insufficient capacity to store the additional water. This drainage is believed to be contributing to watertable rise and salinity in some parts of the Liverpool Plains catchment in northern New South Wales. The effect of land use on deep drainage was investigated by comparing the traditional long fallow system with more intense ‘opportunity cropping’. Long fallowing (2 crops in 3 years) is used to store rainfall in the soil profile but risks substantial deep drainage. Opportunity cropping seeks to lessen this risk by sowing whenever there is sufficient soil moisture. Elements of the water balance and productivity were measured under various farming systems in a field experiment for 4 years in the southern part of the catchment. The experimental results were used to verify APSIM (Agricultural Production Systems Simulator) by comparing them with predictions of production, water storage, and runoff. The verification procedure also involved local farmers and agronomists who assessed the credibility of the predictions and suggested modifications. APSIM provided a realistic simulation of common farming systems in the region and could capture the main hydrological and biological processes. APSIM was then used for long-term (41 years) simulations to predict deep drainage under different systems and extrapolate experimental results. The results showed large differences between agricultural systems mostly because differences in evapotranspiration contributed to differences in profile moisture when it rained. The model predicted that traditional long fallow farming systems (2 crops in 3 years) are quite ‘leaky’, with average annual deep drainage of 34 mm. However, by planting crops in response to the depth of moist soil (opportunity or response cropping), APSIM predicted a much smaller annual drainage rate of 6 mm. Opportunity cropping resulted in overall greater water use and increased production compared with long fallowing. Furthermore, modelling indicated that average annual deep drainage under continuous sorghum (3 mm) is much less than under either long fallow cropping or continuous wheat (39 mm), demonstrating the importance of including summer cropping, as well as increasing cropping frequency, to reducing deep drainage.
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