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Management of grazing lands for the accumulation of soil carbon stocks ([C.sub.S]) has been proposed as an effective way to reduce net greenhouse gas emissions from agriculture. However, there are conflicting reports on the effects of grazing management on soil carbon. Most comparisons have involved some combination of no grazing, rotational grazing and set stocking. In the present study we compared two adjacent commercial grazing systems, distinguished on the basis of inputs and livestock productivity, located on New England basaltic landscapes experiencing a cool temperate climate. The high-intensity (H) system sustains an average stocking rate of 18 dry sheep equivalents (dse) [ha.sup.-1], with a turnoff rate of 9 dse [ha.sup.-1] [year.sup.-1], with high levels of investment in assets, management and fertiliser. The low-intensity (L) system, with less intensive management and half the fertiliser of the H system, sustains a stocking rate of 9 dse [ha.sup.-1], with a turnoff rate of 3 dse [ha.sup.-1] [year.sup.-1], which is slightly higher than the regional average. Pasture biomass production was inferred (back-calculated) from stocking rates and animal feed requirements using published data. From the H and L systems, seven paired landscapes from valley floor to upper hillslopes and plateaux were selected. The seventh included a forest reserve. One hundred and eighty-six undisturbed soil cores (0-0.5 m depth) were assessed for bulk density, total C and N, particulate C and a range of plant nutrients. There were few differences in [C.sub.S], soil pH and nutrient levels between H and L grazing systems. Average [C.sub.S] (0-0.3 m) in pasture soils was 103 Mg [ha.sup.-1], but was higher in the forest soil at 190 Mg [ha.sup.-1]. Regression of [C.sub.S] versus soil mass was a satisfactory method of dealing with the bias introduced by the higher soil bulk density in perennial pasture systems compared with the forest. The similarity of [C.sub.S] in H and L pasture soils was despite inferred net primary production being 1.9-3.6 Mg C [ha.sup.-1] [year.sup.-1] greater in H than L systems, implying higher rates of C turnover in the former. The global warming potential of the inferred annual emissions of C[H.sub.4] and [N.sub.2]O in the H and L systems was equivalent to approximately 19% and 13% of the cycling atmospheric-plant C[O.sub.2] carbon respectively. Additional keywords: grazing animal emissions, particulate soil carbon., Introduction Increasing soil carbon stocks ([C.sub.S]) via changes in management of agricultural land to both increase biomass production and introduce less exploitative practices is an attractive way to reduce net [...]

The impact of cropping on the hydrology and fertility of Vertosols in the northern Darling Basin (average annual rainfall >550 mm) has received much attention, together with the constraints placed on crop growth by naturally occurring subsoil salt stocks. These factors have not been quantified in the drier (450--550 mm), marginal cropping areas to the west. With widespread adoption of zero tillage technology and the potential for large increases in the capture and storage of rainfall in good seasons, mobilisation of salt could be exacerbated should crop water use be constrained by salt toxicity and/or nutrient deficiency. We investigated the size of salt stocks, historic deep drainage, and nutrient depletion under continuous cropping in the Grey and Brown Vertosols of the Walgett and Coonamble districts of north-western NSW. Soils collected from seven paired sites (cropped v. control native vegetation) showed chloride concentrations >500mg/kg within 0-1.2 m, high exchangeable sodium percentage (~30%) at depth and deficiency in phosphorus, manganese and zinc. Soil total nitrogen decreased from an average stock of 4.9 t/ha at a rate of 0.008 t/ha.year under cropping within 0--0.1 m and soil carbon stocks decreased from 39 t/ha by 0.20 t/ha.year within 0-0.5 m.. Despite low rainfall, high evaporation and the large water-holding capacity of the cracking clays, there were significant downward shifts in chloride concentrations under cropping. Estimates of deep drainage under continuous cropping using chloride mass balance, chloride-front displacement and crop water-balance modelling with the Agricultural Production Systems Simulator (APSIM) generally agreed (range 0.1--2% of average annual rainfall). Simulations suggested that deep drainage may be increased 5-10-fold under zero-tillage winter cropping due to enhanced capture of rainfall by zero tillage compared with traditional practices. The associated flushing of salt from the root-zone together with correction of nutrient deficiency would enhance crop water use and productivity. Current methods indicate little storage in the subsoil for future deep drainage and that hydraulic conductivity is very low. Hence, the long-term effects of any increase in drainage rates, due to changes in cropping practices and/ or climate, on the potential for salinisation of groundwater or transient water logging of the surface, are equivocal., Introduction The alluvial cracking clay soils (Vertosols) of the northern reaches of the Darling Basin, eastern Australia, produce ~7 million tonnes (Mt) of wheat, coarse grains and pulses, and ~0.3 [...]