David Whitehead

Intensive dairying emits greenhouse gases and degrades waterways through nutrient runoff and leaching. Trees and shrubs on paddock margins may slow nutrient movement by raising infiltration and by immobilising nutrients in soil carbon. We measured soil carbon along transects running from pasture into three clusters on a pasture-based, intensively managed dairy farm near Lincoln, New Zealand: a mixed New Zealand native planting of nine species (Site MN), a hybrid willow, Salix matsudana × S. alba (Site W), and a planting of Kunzea robusta (kānuka) and Leptospermum scoparium (mānuka) (Site KM). Each planting type occurs at one site, so the study compared three case-study plantings, rather than native and exotic trees in general. Within these plantings, we tested four hypotheses: soil organic carbon (SOC) stock is higher under trees than under pasture (H1); particulate organic carbon (POC) concentration is higher under trees than under pasture (H2); SOC peaks at the dripline (H3); and SOC is higher under exotic trees than under New Zealand natives (H4). Carbon stocks over 0–0.3 m were significantly greater 12 m inside the mixed native planting, at 114 t C ha⁻¹, than at the dripline and in the adjacent pasture, 95 t C ha⁻¹, an increase of 20%. Stocks under willow and under kānuka/mānuka did not differ significantly from pasture. POC varied significantly with location under all three plantings at both 0–0.1 m and 0.1–0.3 m, and mean POC under the trees exceeded that under the adjacent pasture at every site and depth. In the 0–0.1 m layer POC averaged 0.22 to 0.35 g per 100 g of soil under the trees compared to 0.06 to 0.11 g per 100 g of soil under pasture. No planting had its highest carbon at the dripline, and the exotic willow had no more carbon than the natives, so H3 and H4 were not supported in these plantings. Tree clusters on paddock boundaries and marginal land add POC to agricultural landscapes at no cost to soil carbon. Future work should determine whether that carbon immobilises mobile nitrogen and phosphorus.

This research aimed to quantify the chemical composition and rumen degradation kinetics of three local and one imported biowaste using the in sacco nylon bag technique. Triplicate samples of oven‐dried biowastes ground to 4 mm were incubated in the rumens of four cows for 0 h, 2 h, 4 h, 6 h, 12 h, 24 h and 72 h. Bags attached to chains in descending order were placed in the ventral sac of the rumens. The soluble fractions (0 h) of dry matter were lowest for brewer's grains (BG) with 8.4% ± 0.88%, followed by palm kernel expeller (PKE) and potato waste (PW) with 25.4% ± 0.27% and 29.9% ± 1.08%, respectively. Apple pomace (AP) had the highest soluble fraction (54.7% ± 1.11%). For crude protein (CP), the rate of degradation was 9.5% ± 1.0%/h for BG, 5.0% ± 1.1%/h for AP, 0.1% ± 0.0%/h for PKE and 21.1% ± 2.0%/h for PW. These findings demonstrate the variability of local and imported biowastes and provide useful information on degradation kinetics which can be used for formulating diets.

We tested an approach to estimate daily canopy net photosynthesis, A, based on estimates of transpiration, E, using measurements of sap flow and water-use efficiency, ω, by measuring δ¹³C in CO₂ respired from shoots in the canopies of two conifers (Podocarpaceae) native to New Zealand. The trees were planted in adjacent 20-year-old stands with the same soil and environmental conditions. Leaf area index was lower for Dacrycarpus dacrydioides (1.34 m² m⁻²) than for Podocarpus totara (2.01 m² m⁻²) but mean (± standard error) stem diameters were the same at 152 ± 21 mm for D. dacrydioides and 154 ± 25 mm for P. totara. Over a 28-day period, daily A (per unit ground area) ranged almost five-fold but there were no significant differences between species (mean 2.73 ± 1.02 gC m⁻² d⁻¹). This was attributable to higher daily values of E (2.63 ± 0.83 mm d⁻¹) and lower ω (1.35 ± 0.53 gC kg H₂O⁻¹) for D. dacrydioides compared with lower E (1.82 ± 0.72 mm d⁻¹) and higher ω (1.90 ± 0.77 gC kg H₂O⁻¹) for P. totara. We attributed this to higher nitrogen availability and nitrogen concentration per unit foliage area, N_a, and greater exposure to irradiance in the D. dacrydioides canopy compared with P. totara. Our findings support earlier observations that D. dacrydioides is more adapted to sites with poor drainage. In contrast, the high retention of leaf area and maintaining low rates of transpiration by P. totara, resulting in higher water-use efficiency, is an adaptive response to survival in dry conditions. Our findings show that physiological adjustments for two species adapted to different environments led to similar canopy photosynthesis rates when the trees were grown in the same conditions. We demonstrated consistency between whole-tree and more intensive shoot-scale measurements, confirming that integrated approaches are appropriate for comparative estimates of carbon uptake in stands with different species.

This article reviews nine soil carbon interventions to reduce soil carbon losses or increase stocks in grazing management systems, using their potential application in Aotearoa New Zealand (A‐NZ) as a case study. The interventions are classified into three strategies: (1) increasing carbon inputs through deep‐rooting and diverse species grasslands, reducing forage cropping and deferred grazing management; (2) increasing protection of carbon stocks by water table management on organic soils, reducing cropping on organic soils, full inversion tillage grassland renewal, the addition of clay minerals and enhanced rock weathering and (3) the adoption of integrated systems including establishing tree clusters into grasslands and agroforestry. We estimated the land area where these interventions could realistically be implemented in A‐NZ and assessed their potential impact on mitigating national overall agricultural greenhouse gas emissions. The potential impact of each intervention ranged from <1% to 2.5% of national agricultural greenhouse gas emissions over 20 years. However, confidence in these approaches is hindered by a lack of research data where these approaches have been tested for different soil types and conditions. Notably, water table management, which reduces carbon loss from organic soils, was the only intervention that could achieve moderate, short‐ and long‐term impacts with a confidence level assessed as ‘likely’. We conclude that reducing further soil carbon losses and achieving modest increases in soil carbon stocks are possible but will require economic and political incentives that encourage the integration of multiple interventions at the farm scale.

Winery wastewater (WW) is a high-volume biowaste and, in the context of Marlborough and New Zealand wineries, there is a growing recognition of the need to improve current WW disposal systems to mitigate negative environmental impacts. The application of WW to land is a low-cost method of disposal, that could significantly reduce the environmental risk associated with WW directly entering surface and groundwater bodies. This study analysed elemental concentrations in WW and soils from three Marlborough vineyards across their annual vintage to determine the loading rates of nutrients into WW and the subsequent accumulation effects of WW irrigation on receiving soils. The findings showed loading rates of approximately 1.8 t ha⁻¹ yr⁻¹ of sodium within WW and a significant increase in soil sodium concentration and pH, attributed to sodium-based cleaning products. A loading rate of approximately 4 t ha⁻¹ yr⁻¹ of total organic carbon was also identified within WW, however, significant losses in soil carbon, nitrogen, magnesium and calcium concentrations were identified. Focusing efforts to retain key nutrients from WW within soils could provide benefits to New Zealand’s wine industry, facilitating increased biomass production in irrigation plots, thereby increasing biodiversity and potentially generating incentives for vineyard owners to contribute to increasing biomass carbon stocks and offset agricultural greenhouse gas emissions.

Inputs of carbon to soil may be used to stimulate microbial growth and immobilize excess nitrogen from sources such as livestock urine. However, the growth responses of microbial taxa to carbon inputs under conditions of excess soil nitrogen remain poorly understood. Using DNA metabarcoding and a field-based soil lysimeter experiment, we characterised the temporal responses (up to 112 days) of bacterial and fungal communities to a simulated bovine urine event plus inputs of labile carbon (sucrose) at two concentrations. Fungal communities were impacted more strongly than bacterial communities by carbon inputs following the simulated urine event, with more variable responses among taxa. Chytridiomycota and Glomeromycota richness were most negatively affected, and Tremellomycetes richness most positively affected, by carbon inputs. A minority of fungal ASVs had greatly increased proportional abundances in response to carbon, while fungal trophic composition became highly dominated by saprotrophs by the experiment end. Bacterial taxa showed consistent trends of declining (to about 14 days) and recovering (to 112 days) richness in response to urine and carbon inputs, but carbon-related evenness and proportional abundance trends varied between taxa. Proportional abundances of Actinobacteria, Bacteroidetes, Betaproteobacteria, and Gammaproteobacteria increased in response to carbon, whereas proportional abundances of Acidobacteria, candidate division WPS-1, Planctomycetes, Deltaproteobacteria, and Verrucomicrobia decreased. These results show that labile carbon inputs to limit nitrate leaching support the recovery of bacterial communities to bovine urine events but may have long-term impacts on fungal community composition and function, with potential consequences for soil food webs, carbon sequestration, and agricultural productivity.

The biodiversity and structure of deep agricultural soil communities are poorly understood, especially for eukaryotes. Using DNA metabarcoding and co-occurrence networks, we tested whether prokaryote, fungal, protist, and nematode biodiversity declines with increasing depth (0–0.1, 0.3–0.5, and 1.1–1.7m) in pastoral soil; whether deep soil organisms are subsets of those at the surface; and whether multi-kingdom networks become more interconnected with increasing depth. Depth-related richness declines were observed for almost all detected fungal classes, protist phyla, and nematode orders, but only 13 of 25 prokaryote phyla, of which nine had increasing richness with depth. Deep soil communities were not simply subsets of surface communities, with 3.8%–12.2% of eukaryotes and 13.2% of prokaryotes detected only in the deepest samples. Eukaryotes mainly occurred in the upper soil layers whereas prokaryotes were more evenly distributed across depths. Plant-feeding nematodes were most abundant in top soil, whereas bacteria feeders were more abundant in deep soil. Co-occurrence network structure differences suggested that deep soil communities are concentrated around scarce niches of resource availability, in contrast to more spatially homogenous and abundant resources at the surface. Together, these results demonstrate effects of depth on the composition, distribution, and structure of prokaryote and eukaryote soil communities.