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Abstract: For sixty years at Winchmore, South Island, New Zealand (43°48′S, 171°48′E, 160 masl), stoney soils under continuous pasture grazing by sheep have received rainfall (nil irrigation) or rainfall and irrigation as required during summer. This consistently managed, replicated field trial presents a unique opportunity to examine long-term treatment effects on pastoral soil. Samples were recently excavated at intervals to a depth of 1m and the total carbon (C) storage measured. In the irrigated plots, soil C storage (9.1±0.3kgCm−2, mean±standard error, n =3) was significantly less (p <0.05) than in plots receiving rainfall alone (13.4±0.8kgCm−2). We estimated irrigation induced a 36% increase of C inputs to the soil on an annual basis, mostly as litter fall. Using a respiration model based on soil temperature and water content inputs, irrigation was also estimated to have induced a 97% increase in rate of annual C loss to the atmosphere. On this basis, the estimated irrigation effects had reduced C storage by 61% (97–36%), reasonably accounting for the 47% treatment effect determined by soil sampling. [Copyright &y& Elsevier]

Summer drought is a feature of the semi-arid region of central Oregon, USA, where vegetation naturally develops into ponderosa pine (Pinus ponderosa var. Laws) forest. Forest management consists of clearcut harvest and natural regeneration. Soil microbial activity is interconnected with forest processes because substrate quality and availability can be important driving variables. Stand development influences the soil water regime, and water availability may also limit microbial activity. We determined factors limiting litter and mineral soil carbon (C) mineralisation rates in undisturbed old growth and regenerating (hereafter, young) ponderosa pine stands under a semi-arid climate. Mass of litter and dead fine roots did not differ significantly between the stands, but litter substrate quality was different. Young stand litter had significantly higher concentrations of total nitrogen (N), extractable organic N, extractable C, and microbial C and N than that from the old stand, probably because of litter fall from the broadleaved shrub understorey, including the N-fixing species Purshia tridentata (Pursch) DC, that comprised 40% of the young stand’s leaf area. The old stand contained no understorey. For litter samples from the two stands, wetted to 60% of water-holding capacity (WHC), net mineral-N and CO2–C mineralisation rates were similar despite the substrate quality differences. Mineral soil properties at 0–0.1 m depth were similar in the two stands, except for lower CO2–C production in samples from the young stand; at 0.1–0.5 m depth, total C and N and microbial N concentrations were higher in the young stand. Net mineral-N production in field-moist soil, sampled during a typical summer drought and incubated at 25 °C for 56 days, was generally 3–6 mg kg−1 soil at both sites, but increased up to 29 mg kg−1 upon wetting to 60% of water-holding capacity. Over 56-day-long incubations, wetting also increased litter and soil microbial respiration rates by factors of about 500 and 3, respectively. The incubations yielded a proportionality between respiration rate and water content that was supported by in situ measurements of soil respiration in the young stand, before and after irrigation. A hypothetically wet year without soil water deficit caused a 2.5-fold increase in a modelled estimate of the young stand’s annual soil respiration rate. Litter and soil C mineralisation rates in these ponderosa pine forests thus appeared to be limited much more by the availability of water than by a lack of available C or N substrates. [Copyright &y& Elsevier]

For 212 days, including a 117-day-long summer drought with only 45 mm rainfall, fluxes of latent and sensible heat (λE and H, respectively) and carbon dioxide (CO2) were measured by eddy covariance over a tussock grassland in New Zealand. The measurement site was grazed and now has a low leaf area index (∼0.5), dominated by the invasive weed Hieracium, and has a high proportion of bare soil (43%). Energy balance closure was excellent (0.97H+λE)−9 (W m−2)=Ra, r2=0.93, n=8407 half-hourly averages and Ra is the available energy. As the drought progressed, the daily average Bowen ratio (H/λE) increased from 1 to 15 which corresponded with increasing air and surface temperatures and vapour pressure deficit. Maximum evaporation rate was 3.8 mm per day and it averaged 1.1 mm per day, seasonally equivalent to 93% of the rainfall (241 mm) in accordance with mass conservation, but only 47% of the equilibrium evaporation rate. It was estimated that 50% of soil water was utilised before water deficit began to limit λE and, on average, the evaporative fraction λE/Ra declined exponentially with time after rainfall from 0.43 to 0.05 in 15 days.Maximum net carbon dioxide uptake (denoted negative) and efflux (respiration) rates (Re) were −5 and 2 μmol m−2 s−1, respectively. These rates are much less than those for grasslands elsewhere. Soil respiration rate and temperature were exponentially related when water was plentiful, but at the same temperature, respiration declined by a factor of 4 as water content ranged from 0.21 (field capacity) to 0.04 m3 m−3. Soil respiration responded to drought-breaking rainfall and irrigation with a return to maximum rates within 1 day.During spring, when soil water content was >0.10 m3 m−3 (October and November), the daily CO2 flux density (FCO2) was relatively conservative at −1.2 g C m−2 per day. During drought (January–February), the ecosystem became a carbon source with a mean emission flux of 0.70 g C m−2 per day. Twenty days after a drought-breaking rainfall, the ecosystem returned to being a net sink, but by this time the mean daily air temperature was falling and the first frost of the winter (9 April; −1.8 °C) had occurred thus reducing the ability of the ecosystem to respond to the increased soil water content. Water availability severely limits the carbon acquisition of this ecosystem with the result that during summer, net productivity is closely related to both the frequency and the timing of rainfall events. During summer droughts the inter-montane-basin tussock grasslands may be carbon neutral. [Copyright &y& Elsevier]