Pasture Renewal Research Articles

Pasture Performance: Perspectives on Plant Persistence and Renewal in New Zealand Dairy Systems

Pasture systems dominate the landscape of Aotearoa, New Zealand, and are an integral component of sustainable and resilient livestock production. Predicting the response, performance, and dynamics of pasture species and adapting management practices is key to the long-term economic and environmental sustainability and resilience of the agricultural sector. However, there is limited information on the long-term productivity, performance, and persistence of forage cultivars and species for pasture production systems, particularly when linked to grazing and animal performance. Here, we sought to reduce scientific uncertainty, inform modelling efforts, and contribute to a predictive framework for understanding pasture performance, persistence, and renewal. Inter-annual pasture renewal (direct drilling and cultivation) rates vary by region and year, reflecting both opportunity and problem-based drivers, with the highest pasture renewal rates in Waikato and Canterbury on the North and South Island, respectively.

Eddy covariance fluxes of greenhouse gasses observed in a renewed pasture in the southeast of Brazil

The recovery of degraded pastures and the increase in their capacity for storing carbon is a key strategy in Brazil's commitment to meeting its Nationally Determined Contribution (NDC) to the international effort to halt global warming. Here we report the fluxes of CO2, N2O and CH4 gasses and their balances, obtained by the eddy covariance method, from a representative pasture area in southeast Brazil. The measurements covered more than five years, starting with the old degraded pasture, covering the renewal process and then the re-establishment of pasture. The latter only received nitrogen fertilization once after the renewal – the common practice. The net ecosystem carbon balance (NECB), which considered the sum of the net ecosystem exchange (NEEpasture) of CO2 and methane (FCH4pasture) without the cattle respiration and enteric fermentation, as well the export/import of carbon by grazing and as feces (stocking rates of 1.8 to 3.3 AU ha−1), was positive in the old pasture (236 ± 104 g CO2m−2 year−1) representing a source to the atmosphere, but a sink (negative) during the first year of the renewal (-1603 ± 122 g CO2m−2 year−1) when it was fertilized and the recorded rainfall was normal. In the following two years it was neutral (77 ± 93 and 5 ± 100 g CO2m−2 year−1), partially a consequence of the lower rainfall received. In the fourth year it was a sink again (-736 ± 131 g CO2m−2 year−1). On average the cumulative fluxes of CH4 (42 ± 19 g CO2 eq. m−2 year−1) without the animals’ enteric fermentation and N2O (20 ± 10 g CO2 eq. m−2 year−1) were small and positive, being offset by the net carbon gain. The NECB for this site covering the period of measurements was an overall source of 671 ± 254 g CO2m−2.

Changes to soil profile carbon and nutrient distribution following pasture renewal with full inversion tillage

ABSTRACT Full inversion tillage (FIT) at pasture renewal is a management option aiming to increase carbon stocks in long-term pasture, to achieve carbon neutrality. This study investigated the effects of FIT on carbon and nutrient distribution in the soil profile (0–7.5, 7.5–15, 15–22.5 and 22.5–30 cm depths) as well as nutrient uptake, and subsequent fodder crop and/or pasture yields across three pasture renewal trials (Trials 1 and 3: Alfisol; Trial 2: Andisol). These effects of FIT were assessed against standard tillage treatments (no till, shallow till), and non-renewed pasture within 8–18 months post-tillage. FIT changed soil carbon stratification, causing 16%–46% reduction in topsoil (0–7.5 cm) cation exchange capacity across the three trials. However, nutrient levels after FIT remained within recommended ranges for crop and/or pasture growth, avoiding any yield reductions. Topsoil fertility post-FIT depended on original degree of nutrient stratification in the soil profile. At Trial 1, temporary deficiencies caused by low subsoil P and K soil tests pre-FIT were anticipated and corrected with fertiliser nutrients for the following break crop and resown pasture. We conclude that soil testing the cultivation depth prior to FIT at pasture renewal provides the necessary soil test information to manage yield expectations.

Effects of dairy farming management practices on carbon balances in New Zealand’s grazed grasslands: Synthesis from 68 site-years

Globally, the soils underlying managed grasslands are an important reservoir of carbon (C), with the management of the land strongly influencing whether the soils gain, lose or maintain C. Further, improved grassland management, such as rotational grazing and increased sward diversity, are widely promoted to increase soil C sequestration despite limited evidence. Using data from New Zealand’s grazed dairy grasslands, we examined the effect of management practices on C stocks through measurement of the net ecosystem C balance (NECB) across 68 site-years. Management practices tested included year-round rotational grazing, differing pasture sward composition, irrigation, periodic supplemental feed cropping and pasture renewal. Excluding years where supplemental feed cropping occurred, the mean (±95% confidence interval) NECB was −12 ± 30 g C m−2 y−1 (65 site-years; the negative value represents C loss), indicating the soil C stocks of these ecosystems were likely near steady-state. The processes of pasture renewal and the transition between pasture and crops resulted in reduced C inputs from photosynthesis of between −232 and −94 g C m−2. This reduced C input was partially overcome with the application of effluent or manure at the time of cultivation where applied. There was no difference in NECB between irrigated (−26 ± 114 g C m−2 y−1) and non-irrigated (−19 ± 198 g C m−2 y−1) pastures. In general, conventional ryegrass/white clover pastures were more beneficial for C stocks than alternative pasture species and mixes. Periodic cropping for supplemental feed resulted in C losses, with the magnitude depending on crop type and management, although data were limited to three site-years.

The Use of Vegetation Fire in Portugal: Historical Legislative and Normative Analysis

Fire has been a widely used tool in habitat and landscape management, mainly associated with land use dynamics of deforestation, pasture renewal, hunting and reclaiming new agriculture and rangeland areas. Ancient societies followed norms and rules regarding the used of fire. However, as these societies developed and land ownership changed, the conflicts generated by using fire triggered the need to regulate this practice by establishing the first laws. Such laws became of a broader type and narrower application over time until the twenty-first century. This research hypothesises that imposing constraints and regulations on using agro-silvopastoral fire through legislation in Portugal did not discourage its traditional use by communities. Through the historical legislative reconstruction of fire uses from before Portugal’s Foundation until 2021, it was possible to confirm the hypothesis and conclude that a paradigm change is needed by means of setting up an adequate legal framework for the different uses of fire in the land.

Macro-scale spatial modeling reveals the role of soil organic matter quality in CO2 emissions

Carbon dioxide (CO2) emissions from agricultural soils result from the interaction of various factors that regulate the quantity and quality of organic material. Our objective was to assess changes in soil CO2 fluxes following the renewal of degraded pastures using FTIR spectroscopy and a machine learning approach. Additionally, we aimed to understand the spatial distribution of organic compounds obtained from FTIR spectra bands and their correlations with soil attributes. The study was conducted in Selvíria, Mato Grosso do Sul, Brazil, in two areas dedicated to extensive beef cattle farming. Geostatistical grids were established in the study areas to evaluate CO2 emissions, and soil sampling was performed for FTIR spectral analysis and other soil attributes. The semi-quantitative analysis of the CH vibrational region involved spectral deconvolution guided by the positions of the peaks obtained from the second derivative and Gaussian curve fitting. Our results indicate that the relative abundance of compounds related to peaks at 2853 cm−1 and 2923 cm−1 showed linear and spatial correlations with CO2 emissions. Pasture renewal, followed by sorghum cultivation intercropped with Urochloa brizantha, resulted in increased soil pH up to 5.6 and reduced concentrations of compounds related to the aliphatic CH band. This management practice also led to a decrease in CO2 emissions to a range of 0.94 to 0.97 μmol m−2 s−1. The CH2 index, calculated as the ratio of peaks at 2853 cm−1 and 2923 cm−1, exhibited similar changes and could serve as an indicator of soil organic matter loss and CO2 emissions in these systems. Our findings suggest that FTIR organic compounds are spatially dependent and coupled with a machine learning approach is sensitive to distinguish chemical changes in soil organic groups and predict soil CO2 emissions.

Paddock-scale carbon and greenhouse gas budgets in the first year following the renewal of an intensively grazed perennial pasture

Pasture renewal is an important tool commonly used by land managers to maintain a healthy productive pasture sward that can be used for grazing by animals or harvesting. However, there have been few assessments of the impact of pasture renewal on greenhouse gas (GHG) emissions, particularly in grazed pastures. We investigated how pasture renewal impacted the carbon (C), nitrous oxide (N2O) and GHG budgets of an intensively managed rotationally grazed New Zealand pasture by comparing adjacent paddocks with one undergoing pasture renewal. The eddy covariance technique was used to measure carbon dioxide (CO2), N2O and methane (CH4) fluxes which were coupled to measurements and estimates of non-gaseous C flows to calculate the net ecosystem carbon balance and total GHG budget. We measured GHG emissions directly due to pasture renewal of 695 g CO2-eq m-2 during the 66 days between herbicide application and the first grazing of the new pasture sward. Using an adjacent paddock as a control, the net effect of pasture renewal was 358 g CO2-eq m-2, primarily due to an additional grazing event in the non-renewed pasture and the associated CO2 and CH4 emissions associated with digestion of the pasture by the ruminant animals. The pasture renewal emissions were primarily due to C loss with increased N2O emissions accounting for only 10% of total GHG emissions during the renewal period. Total GHG emissions were 2017 g CO2-eq m-2 y-1 for the first year (before pasture renewal), and 1452 and 1894 g CO2-eq m-2 y-1 for the non-renewed and renewed pastures, respectively, in the second (renewal) year. The only significant difference was between the first year and the non-renewed pasture in the second year. We concluded that pasture renewal increased C loss, N2O emissions and total GHG emissions during the renewal period, but when extended to the annual timescale, the effect was insignificant compared to interannual variability.

Pastoral Burning and Its Contribution to the Fire Regime of Alto Minho, Portugal

Alto Minho (in northwestern Iberia) is one of the European regions most affected by fires. Many of these fires originate from rangeland management of Atlantic heathlands, and, while being illegal, often are not actively suppressed. In this study, pastoral fires (autumn-to-spring fires unrecorded by authorities), spring wildfires, and summer wildfires were independently mapped and dated from remote sensing. Alto Minho burned at a mean annual rate of 5.0% of the territory between 2001 and 2020. Pastoral burning totalled 40,788 hectares during the period, accounting for 20% of the total burnt area. Rangeland burning occurs mostly from December to April, the rainiest months that guarantee the conditions for pasture renewal and fire self-extinction. The mean fire return interval of pastoral burning is slightly higher than that of wildfires (13 years vs. 11 years), except in part of the inner mountains where it dominates fire activity. Pastoral fires are more frequent and largely prevail over wildfires in the parishes with higher livestock quantities. Conversely, the largest wildfires and higher summer burnt areas correspond with very low livestock and nearly non-existing pastoral fires. Traditional fire knowledge should not be overlooked by fire management, as it contributes to more sustainable fire regimes and ecosystems.

The carbon balance of a temperate grazed pasture following periodic maize silage cropping depends on climate and management

Periodic cropping of otherwise permanent pastures can lead to large losses of carbon (C), especially if cropped for maize silage. The long-term effect on C stocks due to periodic cropping depends on whether the lost C is recovered before any future cropping events. Accordingly, this study tested the hypothesis that C lost during periodic cropping would be recovered following a return to permanent pasture within a rotationally grazed New Zealand dairy system. In New Zealand, maize silage is often used as supplemental feed, and its production can be part of the pasture renewal process whereby paddocks are cropped for a year or two before re-establishment of a permanent pasture. The net ecosystem carbon balance (NECB) was measured by combining eddy covariance measurements of the CO2 exchange (net ecosystem production; NEP) with measurements and estimates of all other flows of C for four years following the re-establishment of permanent pasture in a paddock previously cropped for maize silage. The NECB (negative values indicate C loss from the ecosystem) of the four years was –13, –21, –52 and −242 g C m-2 for an aggregated total of −328 g C m-2 indicating further C had been lost. November to January (late spring-summer) was identified as crucial for determining the annual NEP and NECB. NEP during November-January was dependent on climate and farm management. Monthly air temperature negatively correlated with NEP, while rainfall positively correlated with NEP for intervals equivalent to the time between grazing events. Increasing the interval between grazing events to around 28 days during November-January was beneficial for increasing NEP, but longer intervals had limited additional benefit. Small day-to-day management decisions such as the application of effluent, grazing rather than harvesting, and increasing the interval between grazing events have the potential to make small but additive gains to the NECB and provide opportunities to recapture lost C (or minimise further losses) following periodic maize silage cropping of permanent pastures. Data from four years, however, suggests little recovery of previously lost C and indicates that the current return period for periodic maize silage cropping (10–15 years per maize crop) might be too short to maintain C stocks.

Place names as indicators of traditional forest management in seminatural landscapes. The case of Cameros (La Rioja, Spain)

Former grazing landscapes, shaped through almost a millennium in the mountains of La Rioja (Spain), have evolved into a mosaic of forests and shrublands once the outstanding transhumance activities collapsed in the mid-20th century due to population decrease. Likewise, agricultural lands were abandoned in a process of ‘rural exodus’ from the countryside to towns within the framework of industrialization and urban sprawl in Spain since the 1960s. However, some place names still indicate these earlier activities, such as the use of fire as a technique for pasture renewal, communal forest thickets [dehesas] temporarily excluded from livestock raising, sustainable management of forests resources (wood, fuel, charcoal), agricultural crops etc. even if nowadays the earlier land uses and covers have changed into recently spread forests. We present a typical selection of examples that show the importance of toponymy in conceiving and explaining the cultural heritage of past seminatural landscapes, particularly of those whose main aspect today is secondary wilderness.

Spring pasture renewal involving full inversion tillage and a summer crop can facilitate soil C storage, improve crop yields and lower N leaching

Soil organic carbon (SOC) sequestration in pastoral soils could offset net greenhouse gas emissions by reducing global atmospheric carbon dioxide (CO2) concentrations and, thereby, slow climate warming. Long-term pastures typically have topsoils (0–10 cm) rich in SOC, with subsoils (10–30 cm) storing less than half as much SOC. In New Zealand, lowland farmers are advised to renew (reseed) their pastures every 7–10 years to improve pasture production. Renewal typically involves desiccating the old pasture followed by shallow tillage (or direct drilling) to establish a short season forage crop as a weed break, then direct drilling new pasture species. Minimum till at renewal maintains the vertical stratification of SOC, limiting the scope to increase SOC stocks under new pasture. During the spring of 2016 and 2017, two independent trials were established, on an Alfisol (trial 1) and on an Andisol (trial 2), to assess the effects of establishing a summer Brassica crop with either full inversion tillage (FIT; 30 cm furrow depth, as a one-off or infrequent (every 25–30 years) management), shallow tillage or no-till (direct drill) on SOC stocks and crop/pasture agronomic performance. In autumn, new pasture species were direct drilled into the stubble of the summer forage crop. Changes in SOC vertical distribution, plant growth, herbage quality (at both trials) and nitrogen (N) leaching (trial 1 only) were monitored. At both trials, FIT effectively buried SOC below 0–10 cm depth and increased crop yields compared to no-till treatment. After re-grassing pasture production was similar among all treatments. In trial 1, N leaching losses were 42% lower under FIT than under no-till. These results highlight the potential agronomic and environmental benefits of pasture renewal including FIT. Economic and N cycling benefits depend on the timely inclusion of a crop phase.

Effects of full inversion tillage during pasture renewal on soil and plant cadmium concentrations: a case study in New Zealand

Context Cadmium (Cd) accumulation is a concern in permanent pasture soils, as it can lead to increased Cd uptake by plants.Aims This study aimed to quantify the effect of full inversion tillage (FIT or ploughing deeper than 30cm), used during pasture renewal, on the redistribution of Cd within the soil profile and on plant Cd concentration.Methods Two field trials (Trial 1, Alfisol; Trial 2, Andisol) were established in New Zealand using contrasting tillage practices (FIT; SIT, shallow tillage; and NT, no tillage) to sow turnips as summer forage crops, followed by autumn re-sowing of perennial ryegrass/white clover pasture.Key results In the Alfisol, no measurable differences (P>0.05) in soil and plant Cd were detected among the tillage treatments. In the Andisol, FIT decreased (P<0.05) total (0.25mg/kg) and extractable soil Cd (0.013mg/kg) in the 0–5cm depth, compared to pre-tillage (0.42 and 0.031mg/kg, respectively). Moreover, at this soil depth, FIT achieved a 52% lower (P=0.034) extractable soil Cd concentration than the ST treatment. In addition, the subsequent new pasture had lower (P=0.007) average Cd concentration following FIT compared to ST (0.03 vs 0.05mg/kg).Conclusions We demonstrated that the use of FIT during pasture renewal is a potential solution to reduce topsoil Cd concentration.Implications The FIT is more effective in soil where total soil Cd concentration or its degree of vertical stratification with depth is relatively high.

Effects of Full Inversion Tillage During Pasture Renewal on Soil and Plant Cadmium Concentrations: A Case Study in New Zealand

Effects of Full Inversion Tillage During Pasture Renewal on Soil and Plant Cadmium Concentrations: A Case Study in New Zealand

Persistence of the yield advantage of perennial ryegrass cultivars: concept, evidence and implications

Pasture persistence can be defined several ways, but a key outcome for farmers is that the yield advantage of a new pasture compared to the pasture it replaced persists for several years after sowing. The concept of persistence of the yield advantage can also be applied to genetic evaluation of cultivars in species such as perennial ryegrass to determine the true value of pasture renewal and cultivar selection. We analysed 8 years of yield and tiller density data from pastures sown to four perennial ryegrass cultivars representing different functional types at two locations, Waikato (non-irrigated) and Canterbury (irrigated). ‘Grasslands Nui’ SE (Nui) was designated as the baseline cultivar. A significant yield advantage over Nui was observed for two cultivars (Alto AR37 and Halo AR37). Peak yield advantage occurred 4 or 5 years post-sowing, then declined by approximately 50% and became nonsignificant by Year 8. The pattern was very similar at both locations. Tiller density data indicated a shift in sward structure over time consistent with size-density trade-offs in the diploid cultivar Alto AR37 but not in the tetraploid cultivar Halo AR37. The implications for economic evaluation systems such as the DairyNZ Forage Value Index are discussed.

The role of Epichloë grass endophytes during pasture renewal

The literature on the importance of Epichloë grass endophytes during pasture renewal is reviewed. Perennial ryegrass endophyte strains such as AR1, NEA2, and Standard Endophyte (SE) as well as tall fescue and meadow fescue endophytes, significantly increase seedling survival at establishment under insect pressure, for example from adult Argentine stem weevil, grass grub, and African black beetle. However, in endophyte-infected ryegrass, insect-derived plant damage increases 10–43 days after sowing despite the presence of endophyte. Insecticidal seed treatments can mitigate the vulnerability to insect predation during this time.

Resilient pastures and sediment: a case study in Hawke's Bay

For many hill-country farms sediment will be a bigger regulatory issue than nitrates over the next decade. A dense, resilient pasture can reduce the risk of insidious sediment loss. Any ecosystem that relies on a few species is fragile. Sowing a single species leads to repeated re-sowing and increasing bare ground to remove competition, increasing the risk of sediment flows. An important issue during regulatory consultation will be establishing a natural, pre-human baseline for forest cover and documenting more recent changes in sediment flows. Hill country cropping and pasture renewal is incompatible with resilient pasture. This is a farmer’s perspective on a diverse and persisting hill country pasture-based system that can make a good return on capital without re-grassing or fodder cropping. Funding of independent research on pasture and fodder systems is essential if farmers are to make good decisions.

Full inversion tillage during pasture renewal to increase soil carbon storage: New Zealand as a case study.

As soils under permanent pasture and grasslands have large topsoil carbon (C) stocks, the scope to sequester additional C may be limited. However, because C in pasture/grassland soils declines with depth, there may be potential to sequester additional C in the subsoil. Data from 247 continuous pasture sites in New Zealand (representing five major soil Orders and ~80% of the grassland area) showed that, on average, the 0.15-0.30m layer contained 25-34t ha-1 less C than the top 0.15m. High-production grazed pastures require periodic renewal (re-seeding) every 7-14years to maintain productivity. Our objective was to assess whether a one-time pasture renewal, involving full inversion tillage (FIT) to a depth of 0.30m, has potential to increase C storage by burying C-rich topsoil and bringing low-C subsoil to the surface where C inputs from pasture production are greatest. Data from the 247 pasture sites were used to model changes in C stocks following FIT pasture renewal by predicting (1) the C accumulation in the new 0-0.15m layer and (2) the decomposition of buried-C in the new 0.15-0.30m layer. In the 20years following FIT pasture renewal, soil C was predicted to increase by an average of 7.3-10.3 (Sedimentary soils) and 9.6-12.7t C ha-1 (Allophanic soils), depending on the assumptions applied. Adoption of FIT for pasture renewal across all suitable soils (2.0-2.6Mha) in New Zealand was predicted to sequester ~20-36 Mt C, sufficient to offset 9.6-17.5% of the country's cumulative greenhouse gas emissions from agriculture over 20years at the current rate of emissions. Given that grasslands account for ~70% of global agricultural land, FIT renewal of pastures or grassland could offer a significant opportunity to sequester soil C and offset greenhouse gas emissions.

Managing eucalyptus trees in agroforestry systems: Productivity parameters and PAR transmittance

Agroforestry systems, in which trees and crops are cultivated in rotation, succession, or association with pastures, are alternatives for the sustainable implementation of agriculture. This study estimated the productive characteristics from eight years old eucalyptus trees in different agroforestry systems and transmission of photosynthetically active solar radiation (PAR). These were composed of a pasture of palisade grass (Urochloa brizantha “BRS Piatã”) and eucalyptus trees (Eucalyptus urograndis “GG100”) planted in April 2011 in single rows 15 m apart with 2 m in-row spacing, totaling 333 trees ha−1. In 2016, half of the trees were thinned, and the spacing was changed to 15 × 4 m. The two systems were then evaluated using an integrated crop-livestock-forestry system (agrosilvopasture with pasture renewal, ICLF) and an integrated livestock-forest system (silvopasture with no pasture renewal, ILF). Each system had 12 paddocks of 5000 m2. In ICLF, pasture was renewed in one-third of each replication of area (two paddocks) per crop year, where the grass was simultaneously sown with corn for silage. Pasture renewal was carried out in the 2013–2014, 2014–2015, 2016–2017, and 2017–2018 growing seasons. Data were collected in April 2016 and June 2019, when 110 trees were harvested to determine wood volume and 28 to gather wood rings and samples of the canopy, roots, and carbon content. These data were used to build the equations for estimating stem volume (m3 tree−1) and tree biomass (kg tree−1). Stem diameter at breast height (DBH, 1.3 m above the ground) and tree height (H) were measured in 10% of the trees in each plot to estimate stem volume and biomass; these were compared by t-test (5%). PAR was measured continuously from 2013 to 2019 at 70 cm aboveground with linear quantum sensors at the four ICLF positions across the tree line. Using these data, equations for volume = exp[(− -10.21 + 1.68 × ln(DBH)+1.29 × ln(H)] and biomass = exp[−3.88 + 2.41*ln(DBH)+0.62 × ln(H)] were built. The stem volume was greater in ICLF (225.7 m3 ha−1) than in ILF (215.2 m3 ha−1) (p = 0.0369). The total biomass was 148.3 Mg ha−1 for ICLF and 141.0 Mg ha−1 for ILF, with no significant differences between systems. The agroforestry systems accumulated 64.5 Mg ha−1 of carbon in tree biomass by eight years after system implementation. The basal area of trees in both agroforestry systems showed a strong relationship with the transmission of PAR to the pasture.

Microbiological and structural quality of Oxisol under pasture renewal systems

ABSTRACT The central Brazilian Savanna biome, known as Cerrado, has a vast area of pastures affected by some degree of degradation, where one of the main challenges is incorporating these areas into a crop production system. This study aimed to evaluate the effect of pasture renewal systems on the microbiological and structural quality of a medium-texture Oxisol. A randomized blocks design was adopted, with four replications and eight pasture renewal systems: 1) soybean/off-season maize/soybean; 2) soybean/maize-grass intercropping/soybean; 3) grass for 10 months and then one soybean crop; 4) grass + rattlepod for 10 months and then one soybean crop; 5) grass for 13 months and then one soybean crop; 6) grass + rattlepod for 13 months and then one soybean crop; 7) one soybean crop; 8) original pasture (control). The microbiological quality was assessed based on soil microbial biomass carbon, soil microbial activity, microbial metabolic quotient - qCO2 and activity of the β-glucosidase enzyme; and the structural quality based on the soil structural quality index. The implementation of pasture renewal systems with grass as a single crop (systems 3 and 5) or intercropped with rattlepod (systems 4 and 6) improves the soil microbiological and structural quality. The pasture renewal system beginning with soybean/off-season maize succession (system 1) is not indicated for the medium-texture soil evaluated in this study.

Large differences in CO2 emissions from two dairy farms on a drained peatland driven by contrasting respiration rates during seasonal dry conditions

Drained peatlands are major sources of CO2 to the atmosphere, yet the effects of land management and hydrological extremes have been little-studied at spatial scales relevant to agricultural enterprises. We measured fluxes of CO2 using the eddy covariance (EC) technique at two adjacent dairy farms on a drained peatland in Aotearoa New Zealand with remaining peat depths 5.5–8 m. One site (SD) had shallow surface drains and mean water table depth (WTD) −657 mm, while the other site (BD) had deep field border drains and mean WTD −838 mm. Net ecosystem CO2 production (NEP) was similar at the two sites when the soils were moist but diverged during late-summer drying, with site BD having 4.56 t C ha−1 greater CO2 emission than site SD over the four-month dry period. Soil drying reduced gross primary production (GPP) at both sites, while ecosystem respiration (ER) was reduced at site SD but not at site BD. The low dry season respiration rates at site SD contributed to near-zero annual NEP, while higher respiration rates at site BD led to annual CO2 loss of −4.95 ± 0.59 t C ha−1 yr−1. Accounting for other imports and exports of carbon, annual net ecosystem carbon balances were −2.23 and −8.47 t C ha−1 yr−1 at sites SD and BD, respectively. It is likely that the contrasting dry season respiration rates resulted from differences in soil physical properties affecting soil moisture vertical redistribution and availability to plants and microbes rather than from the relatively small differences in WTD. These differences could be caused by soil physical disturbances during pasture renewal or paddock recontouring, or time since initial drainage. Therefore, improved soil management might provide practical mitigation against excessive CO2 emissions during dry conditions, including droughts.