A. D. Mackay

Partitioning and translocation of photosynthetically fixed 14C in grazed hill pastures

Abstract Information on carbon (C) flows and transformations in the rhizosphere is vital for understanding soil organic matter dynamics and modelling its turnover. We followed the translocation of photosynthetically fixed C in three hill pastures that varied in their phosphorus (P) fertility, using a 14C-CO2 pulse-labelling chamber technique. Pasture shoot, root and soil samples were taken after 4h, 7 days and 35 days chase periods to examine the fluxes of 14C in the pasture plant-root-soil system. Shoot growth over 35 days amounted to 114, 179 and 182gm–2 at the low (LF), medium (MF) and high (HF) fertility pasture sites, respectively. The standing root biomass extracted from the soil did not differ significantly between sampling periods at any one level of fertility, but was significantly different across the three levels of fertility (1367, 1763 and 2406gm–2 at the LF, MF and HF pastures, respectively). The above- and below-ground partitioning of 14C was found to vary with the length of the chase period and fertility. Although most 14C (74%, 65% and 57% in the LF, MF and HF pastures, respectively) was in the shoot biomass after 4h, significant translocation to roots (23–39%) was also detected. By day 35, about 10% more 14C was partitioned below-ground in the LF pasture compared with the HF pasture. This is consistent with the hypothesis that, at limiting fertility, pasture plants allocate proportionally more resource below-ground for the acquisition of nutrients. In the LF site, with an annual assimilated C of 7064kgha–1, 2600kg was respired, 1861kg remained above-ground in the shoot and 2451kg was translocated to roots. In the HF pasture, of the 17313kgha–1 C assimilated, 7168kg was respired, 5298 remained in the shoot and 4432kg was translocated to the roots. This study provides, for the first time, data on the fluxes and quantities of C partitioned in a grazed pasture. Such data are critical for modelling C turnover and for constructing C budgets for grazed pasture ecosystems.

A nutrient-transfer model to explain the fate of phosphorus and sulphur in a Grazed Hill-Country pasture

Abstract A nutrient-transfer model was developed using a mass balance approach to explain the variations in the amounts of soil phosphorus (P) and sulphur (S) found across a range of slopes and aspects in four 10-ha farmlets located in summer-moist hill-country pastures in the southern North Island of New Zealand. The farmlets were continuously grazed by sheep for a period of 12 years, during which time they received a total of 115, 187, 387, 603 and 126, 201, 405, 630 kg ha −1 of P and S, respectively, as single superphosphate (SSP). The model takes account of the effect of topography and local climate on stock behaviour, herbage accumulation and its nutrient content, pasture utilization and the uneven nutrient return through excreta. The developed P model was reliable. The predictions of the transfer model explained 95% of variation in soil P amounts (0–150 mm depth) between farmlets ( n =4), 89% of variation between slope units ( n =12) and 79% between slope-aspect units ( n =36) across all farmlets. When S input parameters were substituted for P parameters, the model was unable to predict the measured soil S levels in any of the four farmlets. Unlike P, S is subject to losses by leaching from these soils. The differences between predicted and measured soil S amounts were used to estimate S leaching losses and pinpoint areas from where they occurred. The concept of modelling the fate of P in P and S fertilized systems and then using the calculated nutrient-transfer functions of the model to evaluate the fate of S provides invaluable information for developing strategies for improving the efficiency of S fertilizer use.

Effects of fertiliser application on nutrient status and organic matter content of hill soils

Abstract Effects of two fertiliser treatments on soil characteristics were measured during 1972–87, within 10 grazed permanent‐pasture “farmlets”, on steep hill country in southern Hawkes Bay, New Zealand. The low fertiliser (LF) treatment received 125 kg ha‐1 superphosphate p.a., and the high fertiliser (HF) an average 625 kg ha‐1 p.a. for 5 years then 375 kg ha‐1 p.a. subsequently. Ground limestone was applied to HF in 1975 and 1979. Grazing pressure was the same across treatments. Soil fertility decreased with increasing slope of measurement site, and aspect had a less marked effect. Phosphorus initially accumulated mainly in inorganic forms, with organic P accumulating at a slower rate similar to that for organic S. Olsen P status reflected P application regime, although the responsiveness was lower than anticipated. Soil pH in the non‐limed LF treatment increased slightly over time, probably as a consequence of the decline in soil organic C. Soil total N (0–75 mm depth) remained constant in LF, but increased by 19 kg ha‐1 yr‐1 in HF soils. Soil carbon decreased in LF and HF by 200 kg C ha‐1 yr‐1 This suggests that soil organic matter may be decreasing in some New Zealand pastoral environments.

Effects of soil fertility on leaching losses of N, P and C in hill country

Abstract The OVERSEER® nutrient budgets model is increasingly being used by farmers and regional councils to assess N and P inputs and outputs from farms. There are, however, few data for low fertility and high fertility soils in hill country grazed by sheep. Two farmlets at AgResearch Ballantrae, near Woodville, with no‐fertiliser (NF) and 375 kg ha‐1 year‐1 superphosphate high fertiliser (HF) added since 1980, were used to quantitatively estimate N and P cycles under sheep‐grazed pastures at two stages of N saturation. We present data on soils, N and P leaching, and uptake for a trial in which treatments to both farmlets were (a) control, (b) herbicide applied (for broadleaves), and (c) 300 kg N ha‐1 added annually. Trial plots were located on 3–9° slopes in each farmlet. In winter, seepage zones in mini‐catchments in each farmlet generated waters that allowed us to follow nutrient movement from soils to waters. Winter 2006 had above average rainfall, with one intense storm where surface runoff was observed. During other storms, water perched at about 300 mm depth in the subsoil, and moved to seeps solely by subsurface runoff. Mean nitrate‐N concentration in drainage at HF was highest in 2006, as were surface and subsurface runoff, giving a calculated annual loss of nitrate‐N of 44 kg N ha‐1, compared with 20 kg N ha‐1 in 2005 and 27 kg N ha‐1 in 2007. At NF, the losses ranged from 1–2 kg N ha‐1. P losses increased considerably during surface runoff. Mean annual losses in drainage for HF and NF, respectively, were 1.0 and 0.3 kg ha‐1 for dissolved reactive P, 8 and 4 kg ha‐1 for dissolved organic N (DON), and 121 and 228 kg ha‐1 for dissolved organic C (DOC). The DOC/DON ratio was lower at HF (16) than at NF (54). In the plots with 300 kg N added, soil N status increased at NF and losses increased from 2 to 14 kg N ha‐1 year‐1 at 200 mm depth; losses at HF were over 80 kg N ha‐1 year‐1. This suggests soils on these slopes in the HF farmlet can become saturated with N and no longer retain N; soils in the NF farmlet appeared to retain N fertiliser initially, but by years 2 and 3 they appeared to become saturated with N. Gaseous emissions of ammonia, NOx and N2 would also increase as pastures and soils become enriched with N. The OVERSEER® nutrient budgets model gave good predictions of N in waters but underestimated P losses. The pathways of both gaseous losses and immobilisation of N in soils require further study to better quantify the N cycles.

A climate‐driven, soil fertility dependent, pasture production model

Abstract Field trial data relating pasture growth to measures of soil fertility are confounded by many site‐specific environmental factors, particularly the weather. One approach to accommodate this is to express fertiliser responses in terms of relative rather than absolute yields, but this approach places constraints on trial design and is unhelpful when attempting to extrapolate data to estimate actual yields at other sites or in other years. We suggest an alternative approach that includes the soil moisture, and so the effect of climate, as it influences evapotranspiration. The model assumes that pasture growth is proportional to evapotranspiration, and that the proportionality constant (k) depends on soil fertility. Evapotranspiration is calculated from a simple daily soil water balance. Values for k varied from 11 to 19 kg DM ha‐1 mn‐1 of evaporation. The greatest divergence between the measured and modelled production occurred during a prolonged dry period. Possible reasons for this are discussed. With simulated weather data, the model can be used to generate probability‐density functions of pasture production. The advantage of the approach is that prediction of “actual” yield is a very helpful measurement for producers, and more valuable to scientists than relative yield when modelling nutrient cycling. This modelling approach also has potential applications in farm risk management and feed budgeting.

Influence of pastoral fallow on plant root growth and soil physical and chemical characteristics in a hill pasture

Pastoral fallowing over a growing season (October–May) has a profound effect on standing biomass and sward structure, and should have an impact on below ground plant growth and soil biological activities. Two field studies were conducted to compare the effects of pastoral fallow with rotational grazing on root growth and soil physical and chemical properties. Root growth and distribution was altered by pastoral fallowing and there was significantly (P < 0.01) less root biomass at 0–50 mm depth of soil in the fallowed sward than the grazed sward. Compared with the grazed treatment, pastoral fallow increased soil air permeability at 500 mm tension by 38%, saturated hydraulic conductivity by 26%, unsaturated hydraulic conductivity at 20 mm tension by 67% and soil moisture by 10–15%, and reduced soil bulk density by 11%. Fallowing had little effect on soil nutrients both at the end of fallowing, except for small reductions in K and Mineral N levels at 0–75 mm soil depth, and two to three years after fallowing.

Inheritance of phosphorus response in white clover (Trifolium repens L.)

Genotypes of white clover that exhibited divergent responses to P were identified in a glasshouse pot trial. Six high P-responding genotypes were selected from previously identified high P-responding cultivars and 5 low P-responding genotypes were selected from previously identified low P-responding cultivars. These were crossed in a full diallel design without selfing and reciprocals were kept separate. The P-response of progeny lines was compared with parents. High P-response was dominant over low P-response with progeny from crosses between high and low P-response genotypes being similar to the high P-response parent. Reciprocal effects were not significant. The general combining abilities of high P-response genotypes were generally greater than that of the low P-response genotypes, although there were significant specific combining abilities. Narrow sense heritabilities for P response were moderate, 0.46 based on the linear coefficient and 0.33 based on the quadratic coefficient of the fitted response curves.

Application strategies for Sechura phosphate rock use on permanent pasture

At two phosphate (P) responsive sites in hill country the effectiveness of Sechura phosphate rock (SPR) as a direct application P fertilizer for permanent pasture was evaluated. Sechura was applied at two rates, in three different application strategies. The treatments were 16.7 and 50 kgP ha−1 annually, 25 and 75 kgP ha−1 biennially, and 50 and 150 kgP ha−1 triennially giving a total of 50 and 150 kgP ha−1, respectively, over three years. Single superphosphate (SSP) served as the standard P fertilizer. A comparison was also made between SPR and Chatham Rise phosphorite (CRP), another reactive PR. Total pasture and legume production and P uptake by pasture was measured with all fertilizer treatments over a three year period.In the year of application, SPR was as effective as SSP in stimulating total pasture and legume production and P uptake by pasture. This reflects the very reactive nature of this PR. In the second and third years of measurement, SPR did not show superior residual efffects to SSP. The ability of CRP to stimulate legume growth more than SPR in the second year following application demonstrates the danger of generalizing about the residual effects of reactive PR materials. Of the application strategies evaluated, a biennial appplication of 25 kgP ha−1 as SPR maintained legume growth at a higher level than a smaller (16.7 kgP ha−1) annual dressing. The biennial strategy also increased total pasture yield, in addition to legume production to a greater extent in the second and third years than a single (50 kgP ha−1) triennial application.

Soil properties of a widely spaced, planted poplar (Populus deltoides)pasture system in a hill environment

Planting poplars is an effective technology for controlling hill soil erosion in New Zealand pastureland. However, the effects of widely spaced poplars on soil properties are not documented. Soil was sampled from 3 soil depths (0-75, 75-150, and 150-300 mm) in poplar-pasture (PP) and adjacent open pasture (OP) systems. Four sites were examined, 3 unreplicated sites with mature poplars (>29 years, 37-40 stems/ha) and a replicated experiment with immature poplars (5 years, 50-100 stems/ha). Pastures at all sites were dominated by low fertility species and were grazed by sheep and cattle. Pasture species were used in a glasshouse experiment to assess the production potential of the topsoil media (0-20 mm) from the PP and OP soils, and the OP topsoil amended with 5% poplar leaf litter.

Mineralisation and fate of soil sulphur and nitrogen in hill pastures

Abstract A field experiment was conducted for 7 months using mini-lysimeters with ion exchange resin traps to study the amounts of sulphur (S) and nitrogen (N) mineralised from soil organic matter, taken up by pasture and lost by leaching. Four sites were selected on the basis of contrasting fertiliser history and land slope. The fertiliser histories since 1981 for the sites were 125 (LF) or 375 (HF) kg/ha per year single superphosphate (SSP) and the two slope positions were low (LS, 0–12°) and medium (MS, 13–26°). Pasture production at the HF site (8360 ± 250 kg DM/ha) was twice that at the LF site (4986 ± 278 kg DM/ha) and S leaching losses were 7 times greater at the HF site (HF, 15.3 ± 2.9 kg S/ha, LF, 2.1 ± 0.25 kg S/ha). Over the 7 months of measurement, the amounts of S and N mineralised were also greater at the HF site (27.5 ± 4.3 kg S/ha and 251 ± 15.2 kg N/ha, respectively) than the LF site (12 ± 2.1 kg S/ha and 119 ± 8.0 kg N/ha, respectively). Despite the fact that approximately 10 times more ...