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In accord with the Paris Agreement, New Zealand has committed to reducing national greenhouse gas emissions to 30% below 2005 levels by 2030 and further reductions by 2050. Exports of products from agriculture, horticulture, and forestry are important industries, but these industries also contribute 49% of the national greenhouse gas emissions. Research to develop and adopt mitigation practices is underway, but New Zealand also supports the aims of the 4 per 1000 Initiative to increase soil organic carbon (SOC) stocks and remove atmospheric CO2 globally. We used the Soil Carbon Monitoring System statistical model, which is specific to New Zealand, to estimate the effects of land-use change on spatial changes in SOC stocks to a depth of 0.3 m at regional and national scales for the period 1990–2016. During this period, land-use change occurred on only 8.45% of the national land area, and this led to a mean decrease in SOC stocks of 3.3 tC ha−1 on the area where land use changed. This was attributable mainly to the known reductions in SOC stocks that occur when grassland is converted to post-1989 forest, but the effects were dependent on regional land use and slope class. Although the estimated national decrease in SOC stocks of 8.435 MtC was only 0.45% of the total national SOC stocks and 0.35% of the carbon stored in vegetation, our analysis highlights the need for changes to land management practices for New Zealand to increase its SOC stocks.

1. Grazed pasture managers are increasingly being asked to enhance productivity while simultaneously reducing environmental impacts. Using plant traits to design plant communities that optimise ecosystem functions (e.g. productivity, nitrogen retention) may help achieve this. However, trait-function relationships in intensively grazed systems are largely untested. 2. We used a forage diversity experiment, intensively grazed by cows (i.e. 10-12 times per year), to test whether community leaf and root traits were consistent predictors of ecosystem functioning across seasons and dominant species identities. Diversity treatments consisted of adding further species to either a Lolium perenne-Trifolium repens or a Festuca arundinacea-T. repens base mixture. 3. Plant traits were better predictors of functioning in systems dominated by L. perenne than by F. arundinacea. Above-ground productivity, root biomass and soil nitrate concentrations were related to traits in all seasons, but the ability of traits to predict carbon cycling measures, and to a lesser extent, net N mineralisation rates, varied strongly across seasons. 4. Leaf traits were better predictors of functioning than root traits. Despite limited trait breadth, leaf functional trait diversity was correlated with most ecosystem functions in at least one season, but effects were sometimes negative. Trait-function relationships were not always in the expected direction. 5. Synthesis and applications. Our results indicate that manipulating plant community traits has potential to improve some ecosystem functions for some seasons in intensively grazed systems. However, the variable nature of the trait-function relationships found suggests that a deeper understanding of why and when traits relate to ecosystem functioning is required before managers can be confident that using a trait-based approach will consistently improve outcomes.

We determined decadal changes in soil carbon (C) and nitrogen (N) due to different irrigation regimes and phosphorus fertilization of pastures. Archived soil samples (0–75 mm) collected annually from two long‐term trials in New Zealand were analyzed for %C and %N from three P input treatments (ranging from 0 to 376 kg superphosphate ha−1 yr−1, 1952–2009) and three irrigation treatments (unirrigated and irrigated when soil moisture content fell below either 10 or 20%, 1959–2002). In the fertilizer trial, soil C increased linearly from 2.7 to 4.2%, and there was no difference in rates of increase in C between treatments, despite much greater aboveground production when P was added. This lack of difference was attributed to higher stocking rates on treatments with higher production, and to the possibility that root inputs (which differed less between treatments) was a more important control of soil C accumulation. Nitrogen (%) was lower in the unfertilized than fertilized treatments due to lower clover N fixation, which was constrained by P availability. Soil C (%) was significantly greater in the unirrigated treatment than the irrigated treatments throughout the trial. Aboveground production was much greater in the irrigated than dryland treatment but root biomass was lower. Irrigation must have increased C and N losses, possibly via increased respiration rates during seasonally dry periods. Our study showed that P fertilizer application did not result in an increase in surface soil C and that flood irrigation resulted in a constrained increase in surface soil C content.

Mitigation practices for nitrogen leaching losses from livestock agriculture are needed to protect freshwater quality and increase the efficiency of agricultural production. Within New Zealand, the most common pasture type is a two-species mix of perennial ryegrass ( Lolium perenne ) and white clover ( Trifolium repens ). Ecological theory suggests that increasing species and functional diversity improves ecosystem function, including nitrogen (N) retention. Use of more diverse pasture types, including a mix of pasture grasses, legumes and other forbs, particularly plantain ( Plantago lanceolata ), with functional traits, including winter activity, deep-rooting, N fixation, and biological inhibition of nitrification in the soil, is a potential mitigation practice that requires further verification with long-term field measurements. Here we utilize a network of large lysimeters to make field-based measurements of N leaching from 5–8 species diverse pasture, including plantain, under a range of soil, climate and management conditions, for comparison with losses from traditional ryegrass-clover pasture. Over 3 years of measurements, leaching from fully established diverse pasture was 2–80 kg N ha −1  y −1 . No differences were observed in dry matter production or N leaching of diverse pasture compared to ryegrass-clover lysimeters. Large losses, up to 120 kg N ha −1 , were observed during periods when pasture was not fully established, including cultivation and sowing of new pasture, depending on season. Timing of management activities could be optimized to minimize these losses. These data provide critical assessment of diverse pasture as a mitigation approach for reducing N losses. Further work on diverse pastures should include higher diversity mixes as well as consideration of animal mediated effects of diverse pasture diets on N inputs.

Agricultural production systems face increasing threats from more frequent and extreme weather fluctuations associated with global climate change. While there is mounting evidence that increased plant community diversity can reduce the variability of ecosystem functions (such as primary productivity) in the face of environmental fluctuation, there has been little work testing whether this is true for intensively managed agricultural systems. Using statistical modeling techniques to fit environment–productivity relationships offers an efficient means of leveraging hard‐won experimental data to compare the potential variability of different mixtures across a wide range of environmental contexts. We used data from two multiyear field experiments to fit climate–soil–productivity models for two pasture mixtures under intensive grazing—one composed of two drought‐sensitive species (standard), and an eight‐species mixture including several drought‐resistant species (complex). We then used these models to undertake a scoping study estimating the mean and coefficient of variation (CV) of annual productivity for long‐term climate data covering all New Zealand on soils with low, medium, or high water‐holding capacity. Our results suggest that the complex mixture is likely to have consistently lower CV in productivity, irrespective of soil type or climate regime. Predicted differences in mean annual productivity between mixtures were strongly influenced by soil type and were closely linked to mean annual soil water availability across all soil types. Differences in the CV of productivity were only strongly related to interannual variance in water availability for the lowest water‐holding capacity soil. Our results show that there is considerable scope for mixtures including drought‐tolerant species to enhance certainty in intensive pastoral systems. This provides justification for investing resources in a large‐scale distributed experiment involving many sites under different environmental contexts to confirm these findings. We test whether increasing diversity enhances stability in pasture production. Models were used to scale up experimental data to national predictions. The more diverse mixture had greater stability across soil–climate combinations

Overyielding, the primary metric for assessing biodiversity effects on ecosystem functions, is often partitioned into “complementarity” and “selection” components, but this reveals nothing about the role of increased resource use, resource-use efficiency, or trait plasticity. We obtained multiple overyielding values by comparing productivity in a five-species mixture to expected values from its component monocultures at a) six levels of nitrogen addition (spanning 0–500 kg N ha⁻¹ year⁻¹) and b) across four seasons. We also measured light, water, and nitrogen use, resource-use efficiency, and three functional traits—leaf nitrogen content, specific leaf area, and leaf area ratio—n mixtures and monocultures. We found strong evidence for non-transgressive overyielding. This was strongest in spring, with mixture productivity exceeding expected values by 20 kg dry matter ha⁻¹ day⁻¹. Peak overyielding was driven by enhanced complementarity, with the two non-N₂-fixing forb species far exceeding expected productivity in mixtures. Peak overyielding also coincided with higher water use in the mixture than for any monoculture, and enhanced mixture-resource-use efficiency. There was only weak evidence that trait plasticity influenced overyielding or resource use. Our findings suggest that when complementarity drives overyielding in grassland mixtures, and this is made possible both by increased water use and enhanced efficiency in water, nitrogen, and light use. Our results also suggest that mixtures offer a viable compromise between productivity, resource-use efficiency, and reduced environmental impacts (i.e., nitrate leaching) from intensive agriculture.

Plant functional traits are thought to drive variation in primary productivity. However, there is a lack of work examining how dominant species identity affects trait–productivity relationships. The productivity of 12 pasture mixtures was determined in a 3‐year field experiment. The mixtures were based on either the winter‐active ryegrass (Lolium perenne) or winter‐dormant tall fescue (Festuca arundinacea). Different mixtures were obtained by adding forb, legume, and grass species that differ in key leaf economics spectrum (LES) traits to the basic two‐species dominant grass–white clover (Trifolium repens) mixtures. We tested for correlations between community‐weighted mean (CWM) trait values, functional diversity, and productivity across all plots and within those based on either ryegrass or tall fescue. The winter‐dormant forb species (chicory and plantain) had leaf traits consistent with high relative growth rates both per unit leaf area (high leaf thickness) and per unit leaf dry weight (low leaf dry matter content). Together, the two forb species achieved reasonable abundance when grown with either base grass (means of 36% and 53% of total biomass, respectively, with ryegrass tall fescue), but they competed much more strongly with tall fescue than with ryegrass. Consequently, they had a net negative impact on productivity when grown with tall fescue, and a net positive effect when grown with ryegrass. Strongly significant relationships between productivity and CWM values for LES traits were observed across ryegrass‐based mixtures, but not across tall fescue‐based mixtures. Functional diversity did not have a significant positive effect on productivity for any of the traits. The results show dominant species identity can strongly modify trait–productivity relationships in intensively grazed pastures. This was due to differences in the intensity of competition between dominant species and additional species, suggesting that resource‐use complementarity is a necessary prerequisite for trait–productivity relationships. Inclusion of forbs (Plantago lanceolata and Chicorium intybus) in ryegrass‐based mixtures increased productivity while their inclusion in tall fescue‐based mixtures decreased productivity. This is most likely due to a lack of seasonal complementarity between the forbs and tall fescue.

Questions: How do the traits of pastoral species respond to growth in mixture, nitrogen addition and season? What are the impacts of trait plasticity on community aggregate trait values? Study site: A large-scale field experiment on intensively managed dairy pastures in New Zealand. Methods: We measured traits linked to rate of return on investment in leaves – leaf nitrogen content (leaf N) and specific leaf area (SLA) – and biomass investment in leaf area – leaf area ratio (LAR). We collected trait data for 5 pasture species (one grass, two forbs, and two N2-fixing legumes) grown in monoculture or a five-species mixture across three levels of nitrogen (N) addition in four seasons. For each species in each season we tested for significant effects of growth in mixture, N addition, and their interaction. We calculated community-weighted mean (CWM) values in mixture plots using traits collected either from mixtures or monocultures. We tested for significant mixture and N addition effects on CWM, and for significant interactions between mixture and N addition. Results: SLA and LAR for all non-N2-fixers were significantly higher in spring, summer or autumn, and never significantly lower in mixture than in monoculture. All three non-N2-fixers experienced higher leaf N in mixture during summer, but two species had significantly lower leaf N in either winter or autumn. Mixture effects on CWM values for all three traits were negative in winter and positive in either spring or summer. Conclusions: The direction of trait plasticity effects on community level trait means was highly seasonally dependent.

I must again write to you about a leader with which I take exception (the last being in the 1970s, over Mudging and Fudging no less, which you graciously published). As an Englishmen and Guardian reader who has spent significant time living in both the US and Eng land, your comments (In praise of...