Soil and solution chemistry under pasture and radiata pine in New Zealand

The conversion of hill country pasture to exotic forest plantations is occurring rapidly (70,000 ha yr^–1) in New Zealand. Impacts of this land-use change on soil properties, soil fertility, and water quality are only beginning to be investigated. This study examines the effects of radiata pine (Pinus radiata) on soil and soil solution chemistry, in a region of low atmospheric pollution, 20 years after plantation establishment, assuming that the pasture and pine research sites had comparable soil properties before planting pine. The primary effects of conversion on soil chemistry were a decrease of organic carbon in the mineral soil that was balanced by an accumulation of the surface litter layer, a decrease in soil N, soil acidification, and increased pools of exchangeable Mg, K, and Na. Soil solution studies revealed a large input of sea salts by enhanced canopy capture of sea salts that contributed to much larger solute concentrations and elemental fluxes in the pine soil. Sea salts appear to accumulate in the micropores of pine soil during the dry summer period and are slowly released to macropore flow during the rainy season. This results in a progressive decrease in solute concentrations over the period of active leaching. While chloride originating from sea salt deposition was the dominant anion in the pine soil, bicarbonate originating from root and microbial respiration was the dominant anion in the pasture soil. Carbon dioxide concentrations in the soil atmosphere were 12.5-fold greater in the pasture soil than in the pine soil due to greater rates of root and microbial respiration and to slower diffusion rates resulting from wetter soil conditions in the pasture. Although elemental fluxes from the upper 20 cm of the soil profile were substantially greater in the pine soil, these losses were compensated for by increased elemental inputs resulting from nutrient cycling and enhanced canopy capture of sea salts.     

Mechanisms for changes in soil carbon storage with pasture to Pinus radiata land-use change

In this study, we simulated pasture to Pinus radiata land‐use change with the Generic Decomposition And Yield (G'DAY) ecosystem model to examine mechanisms responsible for the change in soil carbon (C) under pine. We parameterized the model for paired sites in New Zealand. Our simulations successfully reproduced empirical trends in ecosystem productivity and soil inorganic nitrogen (N), and modeled an increase in soil C and a small decline in soil N after 30 years under pine. We determined the mechanisms contributing to soil C change based on an established hypothesis that attributes increases in soil C storage to three main factors: increased ecosystem N inputs relative to outputs, increased C/N ratios in plant and soil, or a shift of N from plant to soil. The mechanisms we attributed to the simulated increase in soil C under pine were increased soil C inputs through tree litterfall, and an increase in the soil C/N ratio. In the first 7 years following pine establishment, a decline in soil C was simulated; this was matched by a decline in soil N. The simulated longer‐term increase in soil C with afforestation by pine contrasts with results from published field studies, which show either a decline or no change in soil C under pine. The discrepancy between measured and simulated changes in soil C was attributed to the G'DAY model overestimating the transfer of litter C into the mineral soil.     

The effects of elevated CO2 on symbiotic N2 fixation: a link between the carbon and nitrogen cycles in grassland ecosystems

The response of plants to elevated CO₂ is dependent on the availability of nutrients, especially nitrogen. It is generally accepted that an increase in the atmospheric CO₂ concentration increases the C:N ratio of plant residues and exudates. This promotes temporary N-immobilization which might, in turn, reduce the availability of soil nitrogen. In addition, both a CO₂ stimulated increase in plant growth (thus requiring more nitrogen) and an increased N demand for the decomposition of soil residues with a large C:N will result under elevated CO₂ in a larger N-sink of the whole grassland ecosystem. One way to maintain the balance between the C and N cycles in elevated CO₂ would be to increase N-import to the grassland ecosystem through symbiotic N₂ fixation. Whether this might happen in the context of temperate ecosystems is discussed, by assessing the following hypothesis: i) symbiotic N₂ fixation in legumes will be enhanced under elevated CO₂, ii) this enhancement of N₂ fixation will result in a larger N-input to the grassland ecosystem, and iii) a larger N-input will allow the sequestration of additional carbon, either above or below-ground, into the ecosystem. Data from long-term experiments with model grassland ecosystems, consisting of monocultures or mixtures of perennial ryegrass and white clover, grown under elevated CO₂ under free-air or field-like conditions, supports the first two hypothesis, since: i) both the percentage and the amount of fixed N increases in white clover grown under elevated CO₂, ii) the contribution of fixed N to the nitrogen nutrition of the mixed grass also increases in elevated CO₂. Concerning the third hypothesis, an increased nitrogen input to the grassland ecosystem from N₂ fixation usually promotes shoot growth (above-ground C storage) in elevated CO₂. However, the consequences of this larger N input under elevated CO₂ on the below-ground carbon fluxes are not fully understood. On one hand, the positive effect of elevated CO₂ on the quantity of plant residues might be overwhelming and lead to an increased long-term below-ground C storage; on the other hand, the enhancement of the decomposition process by the N-rich legume material might favour carbon turn-over and, hence, limit the storage of below-ground carbon.     

15N natural abundance in forest and pasture soils of the Brazilian Amazon Basin

The natural abundance of ¹⁵N was examined in soil profiles from forests and pastures of the Brazilian Amazon Basin to compare tropical forests on a variety of soil types and to investigate changes in the sources of nitrogen to soils following deforestation for cattle ranching. Six sites in the state of Rondônia, two sites in Pará and one in Amazonas were studied. All sites except one were chronosequences and contained native forest and one or more pastures ranging from 2 to 27 years old. Forest soil ¹⁵N values to a depth of 1 m ranged from 8 to 23 and were higher than values typically found in temperate forests. A general pattern of increasing ¹⁵N values with depth near the soil surface was broadly similar to patterns in other forests but a decrease in ¹⁵N values in many forest profiles between 20 and 40 cm suggests that illuviation of ¹⁵N-depleted nitrate may influence total soil ¹⁵N values in deeper soil where total N concentrations are low. In four chronosequences in Rondônia, the ¹⁵N values of surface soil from pastures were lower than in the original forest and ¹⁵N values were increasingly depleted in older pastures. Inputs of atmospheric N by dinitrogen fixation could be an important N source in these pastures. Other pastures in Amazonas and Pará and Rondônia showed no consistent change from forest values. The extent of fractionation that leads to ¹⁵N enrichment in soils was broadly similar over a wide range of soil textures and indicated that similar processes control N fractionation and loss under tropical forest over a broad geographic region. Forest ¹⁵N profiles were consistent with conceptual models that explain enrichment of soil ¹⁵N values by selective loss of ¹⁴N during nitrification and denitrification.     

Soil restoration under pasture after topsoil removal: Some factors influencing C and N mineralization and measurements of microbial biomass

Factors influencing rates of C and N mineralization of soil and plant materials, and the reliability of different procedures for estimating microbial biomass, were measured in a soil (Typic Dystrochrept) that had been restored under grazed pasture in a temperate environment for 10–11 years after 20 cm of the original topsoil had been removed by stripping. Rates of net N mineralization were appreciably lower, but CO₂-C production higher, in the stripped than in the unstripped soil. These activities were not influenced directly by levels of soil mineral-N, but they were influenced by differences in plant composition. Herbage and litter, and roots, from the stripped plots were generally mineralized more readily to CO₂-C, but more slowly to net mineral-N, than were the corresponding materials from the unstripped plots. Rates of mineralization of herbage and litter, or roots, were mainly indistinguishable in stripped and unstripped soil, whereas rates of mineralization of all standing dead material were lower in stripped soil. Measurements of extractable-C flush, and of CO₂-C flush (using a fumigated soil control) and mineral-N flush by fumigation-incubation procedures, indicated that microbial biomass in stripped soil had recovered to at least 88 percent of the levels in unstripped soil. Substrate-induced respiration also generally indicated high levels of recovery of microbial biomass. The fumigation-incubation procedure appeared to under-estimate microbial biomass markedly in stripped soil when unfumigated soil controls were used; the used of a large soil inoculum (20 percent w/w) only sometimes overcame this problem. Possible reasons for apparent anomalies in estimation of microbial C are discussed.     

Trichothecene production by Fusarium species isolated from grain and pasture throughout New Zealand

Liquid cultures of 200 Fusarium isolates selected to represent the most common species found in autumn pasture (70 isolates) and in grain (130 isolates) grown in New Zealand were analysed for trichothecenes and related compounds. Production of butenolide, cyclonerodiol derivatives and culmorins was also measured. The principal trichothecenes produced were derivatives of either nivalenol (NIV), deoxynivalenol (DON) or scirpentriol (Sctol), in order of frequency. The principal trichothecene producing species were F. crookwellense, F. culmorum and F. graminearum. Isolates of the first two species were predominantly NIV-chemotypes with one or two isolates respectively as Sctol-chemotypes. F. graminearum showed equal quantities of NIV- and DON-chemotypes, with the DON-chemotypes producing primarily 15-acetyldeoxynivalenol (15-ADON).     

Nitrogen inputs and losses from New Zealand dairy farmlets, as affected by nitrogen fertilizer application: year one

Inputs and losses of nitrogen (N) were determined in dairy cow farmlets receiving 0, 225 or 360 kg N ha^-1 (in split applications as urea) in the first year of a large grazing experiment near Hamilton, New Zealand. Cows grazed perennial ryegrass/white clover pastures all year round on a free-draining soil. N₂ fixation was estimated (using ¹⁵N dilution) to be 212, 165 and 74 kg N ha^-1 yr^-1 in the 0, 225 and 360 N treatments, respectively. The intermediate N rate had little effect on clover growth during spring but favoured more total pasture cover in summer and autumn, thereby reducing overgrazing and resulting in 140% more clover growth during the latter period.Removal of N in milk was 76,89 and 92 kg N ha^-1 in the 0, 225 and 360 N treatments, respectively. Denitrification losses were low (7–14 kg N ha^-1 yr^-1), increased with N application, and occurred predominantly during winter. Ammonia volatilization was estimated by micrometeorological mass balance at 15, 45 and 63 kg N ha^-1 yr^-1 in the 0, 225 and 360 N treatments, respectively. Most of the increase in ammonia loss was attributed to direct loss after application of the urea fertilizer.Leaching of nitrate was estimated (using ceramic cup samplers at 1 m soil depth, in conjunction with lysimeters) to be 13, 18 and 31 kg N ha^-1 yr^-1 in a year of relatively low rainfall (990 mm yr^-1) and drainage (170–210 mm yr^-1). Drainage was lower in the N fertilized treatments and this was attributed to enhanced evapotranspiration associated with increased grass growth.Nitrate-N concentrations in leachates increased gradually over time to 30 mg L^-1 in the 360 N treatment whereas there was little temporal variation evident in the 0 (mean 6.4 mg L^-1) and 225 (mean 10.1 mg L^-1) N treatments. Thus, the 360 N treatment had a major effect by greatly reducing N₂ fixation and increasing N losses, whereas the 225 N treatment had little effect on N₂ fixation or on nitrate leaching. However, these results refer to the first year of the experiment and further measurements over time will determine the longer-term effects of these treatments on N inputs, transformations and losses.     

Global climate change and soil carbon stocks; predictions from two contrasting models for the turnover of organic carbon in soil

Enhanced release of CO₂ to the atmosphere from soil organic carbon as a result of increased temperatures may lead to a positive feedback between climate change and the carbon cycle, resulting in much higher CO₂ levels and accelerated global warming. However, the magnitude of this effect is uncertain and critically dependent on how the decomposition of soil organic C (heterotrophic respiration) responds to changes in climate. Previous studies with the Hadley Centre's coupled climate–carbon cycle general circulation model (GCM) (HadCM3LC) used a simple, single‐pool soil carbon model to simulate the response. Here we present results from numerical simulations that use the more sophisticated ‘RothC’ multipool soil carbon model, driven with the same climate data. The results show strong similarities in the behaviour of the two models, although RothC tends to simulate slightly smaller changes in global soil carbon stocks for the same forcing. RothC simulates global soil carbon stocks decreasing by 54 Gt C by 2100 in a climate change simulation compared with an 80 Gt C decrease in HadCM3LC. The multipool carbon dynamics of RothC cause it to exhibit a slower magnitude of transient response to both increased organic carbon inputs and changes in climate. We conclude that the projection of a positive feedback between climate and carbon cycle is robust, but the magnitude of the feedback is dependent on the structure of the soil carbon model.     

Direct seeding of indigenous tree and shrub species into New Zealand hill country pasture

Summary Our study aimed to examine the applicability of a native plant restoration technique involving close grazing, blanket herbicide spraying and mob stocking (to trample the seed into the soil) for establishing New Zealand native tree and shrub species into exotic hill country pasture. This approach represents an alternative to the usual spaced-planting of seedlings in the context of an indigenous vegetation restoration programme. Field-collected seed of Cabbage Tree ( Cordyline australis ), Kahikatea ( Dacrycarpus dacrydioides ), Kanuka ( Kunzea ericoides ), Karamu ( Coprosma robusta ), Kohuhu ( Pittosporum tenuifolium ), Koromiko ( Hebe stricta ), and Manuka ( Leptospermum scoparium ) were germination tested in the laboratory, and broadcast sown in two field experiments in spring 2001 and autumn 2002. The spring-sown experiment tested the effect of mob stocking and the autumn-sown experiment tested the effect of mob stocking and sowing rate. Laboratory germination varied widely among species (0–88%). In the field, the two shrubs, Koromiko and Karamu, established densities of up to 5 and 1.2 plants/m², respectively, after 2.5 years. Autumn emergence for these two species strongly related to sowing rate. Three tree species, Cabbage Tree, Kahikatea and Manuka, emerged in the field but did not survive beyond 6 months. Mob stocking was effective in spring but not autumn sowing, implicating soil moisture content as an important factor in germination success. The study demonstrated that the direct seeding technique was successful for two shrub species, highlighting the potential for development of a cost-effective native plant restoration approach.     

The effect of climate and cultivation on soil organic C and N

Here, we investigate the response of soil organic carbon (SOC) and soil organic nitrogen (SON) to cultivation within two different climatic regimes by comparing large soil data sets from India and the Great Plains. Multiple regression models for both regions show that SOC and SON, as well as C/N ratios, increase with decreasing temperatures and increasing precipitation, trends also noted in soil data organized by Holdridge life zones. The calculated difference between natural and cultivated soils in India revealed that the greatest absolute SOC and SON losses occurred in regions of low temperatures and high precipitation, while the C/N ratio decreased during cultivation only with decreasing temperature. In India, the fractional loss of SOC relative to undisturbed soils increases with decreasing temperature whereas, in the Great Plains, it increases with increasing precipitation. Also, the fractional loss of SOC increased in India with increasing amounts of original C, whereas no relationship between fractional loss and original C was noted for the Great Plains. The differential response of each region to cultivation is hypothesized to be due to differences in both climate and management practices (crop cycles, fertilization). These findings suggest that estimates of soil C loss due to cultivation should be based on an array of factors, and that it is unlikely that a constant relative C loss occurs in any region.     

Diurnal changes in nitric oxide emissions from conventional tillage and pasture sites in the Amazon Basin: influence of soil temperature

We have investigated a subset of restoration practices applied to a degraded pasture at Fazenda Nova Vida, a 22000 ha cattle ranch in Rond^onia, Brazil. Nitric oxide (NO) and carbon dioxide (CO₂) emissions from soils were measured in conventional tillage and current pasture sites to assess N and C losses. Mean daily NO emissions from tilled plots were at least twice those from the pasture. Nitric oxide emissions from the tilled sites showed a strong diurnal pattern, while those from the pasture sites did not. Mean daytime NO emissions from the tilled sites were 9.7 g NO-N m^–2 h^–1, while mean nighttime emissions were 29.7 g NO-N m^–2 h^–1. In the pasture sites, NO emissions were 7.6 g NO-N m^–2 h^–1 during the day, and 7.7 g NO-N m^–2 h^–1 at night. Surface soil temperature was a good inverse predictor (r ²=0.75) of NO emissions from the tilled sites. Carbon dioxide emissions from the tilled sites were generally larger than CO₂ emissions from the pasture sites. The mean CO₂ emission rate from the tilled sites was 179 mg C m^–2 h^–1, while it was 123 mg C m^–2 h^–1 from the pasture sites. There was no distinct diurnal pattern for CO₂ emissions. We found that the very high temperatures measured at the soil surface in the tillage plots, in the range of 40–45°C, reduced the rate of NO emission. The reduction in NO emissions may be because of the sensitivity of autotrophic nitrifiers to high temperatures. This study provides insights on how land-use change may alter regional NO fluxes by exposing certain microbial communities to extreme environmental conditions. Future studies of NO emissions in tropical agricultural systems where soils are bare for extend periods need to make diurnal measurements or the daily fluxes will be substantially underestimated.     

Terrestrial carbon storage resulting from CO2 and nitrogen fertilization in temperate grasslands

Abstract. A temperate grassland model has been used to simulate carbon sequestration under various environmental conditions. The results suggest that the CO₂ and nitrogen fertilization that has occurred may contribute appreciably to the so-called missing carbon sink, which it has been suggested must exist to balance the global carbon budget.     

Temporal changes in C,P and N concentrations in soil solution following application of synthetic sheep urine to a soil under grass

We have determined the temporal changes in the concentration of dissolved organic carbon (DOC) and P and N components in soil solution following application of synthetic sheep urine (500 kg N ha^-1) to a brown forest soil in boxes sown with Agrostis capillaris. Three contrasting defoliation treatments (no cutting, single cut before urine application and regular cutting twice per week) plus a fallow soil were studied. The synthetic urine contained ¹⁵N labelled urea and was P-free. Intact soil cores were taken after 2, 7, 14, 21 and 56 d and centrifuged to obtain soil solution. The urea in the synthetic urine was rapidly hydrolysed in the soil, increasing soil solution pH, DOC and total dissolved phosphorus (TDP) concentrations. For the regularly defoliated sward, DOC and P reached maximum concentrations (4000 mg DOC L^-1 and 59 mg TDP L^-1) on day 7. From their peak values, pH and DOC and P concentrations generally decreased with time and at day 56 were near those of the control. Concentrations of NH₄ ⁺ and NO₃ ^- in the no-urine treatments fluctuated and the greatest treatment differences were between the fallow soil and the soil sown with grass. Adding synthetic urine increased NH₄ ⁺ concentrations during the first week, but NO₃ ^- concentrations decreased. This was consistent with the ¹⁵N labelling of the NO₃ ^- pool which required 3 weeks to reach that of ¹⁵NH₄ ⁺. Dissolved organic nitrogen (DON) reached a maximum value at day 7 with a concentration of 409 mg N L^-1. The DON in soil solution contained no detectable amounts of ¹⁵N label indicating that it was derived from sources in the soil. Differences in soil solution composition related to the effect of the other cutting treatments and the fallow treatment were small compared to the effect of synthetic urine addition. This revised version was published online in June 2006 with corrections to the Cover Date.     

The variability in total and extractable soil phosphorus under a grazed pasture

A comparison of the total soil phosphorus (P) and extractable soil P between 224 samples of topsoil from an area of ~27 m² within a grazed, established grass/clover sward has been made. The values of total soil P displayed an approximately normal distribution around a mean concentration of 1264 mg P kg-1 and were positively correlated to acetic-acid-extractable P which accounted for <2% of the total soil P. The amount of total water-extractable P was much smaller (~0.4% of total soil P) and was not significantly correlated with either the concentration of total soil P or acetic-acid-extractable P. A variable proportion (from less than 5 to 60%) of the total water-extractable P was present in a non-molybdate-reactive form, and there was no apparent relationship between these molybdate-reactive and molybdate-unreactive forms. All variograms showed a positive intercept on the ordinate. For acetic-acid-extractable P, the greatest proportion of variance was attributable to the molybdate-reactive P fraction, while it was equally proportioned between molybdate-reactive and -unreactive P forms in water extracts. The greatest variance usually occurred at the maximum sampling distance (18 m). However, even at the smallest distance (11 cm) the variability in total acetic-extractable P was 2.35 mg P kg-1 and water-extractable P was 0.45 mg P kg-1. Therefore the roots of individual plants within the studied pasture may encounter considerable variability in the concentration of available phosphorus. The potential variability of phosphorus found between rooting zones of different individual plants was greater than that likely to be encountered within the area exploited by any one individual root system.     

Comparison of initial wetting pattern,nutrient concentrations in soil solution and the fate of15N-labelled urine in sheep and cattle urine patch areas of pasture soil

Three field experiments were carried out to compare cattle and sheep urine patches in relation to (i) initial wetting pattern and volume of soil affected, (ii) soil solution ionic composition and (iii) the fate of¹⁵N-labelled urine in the soil over the winter period. The distribution of Br^– (used as a urine tracer) across the soil surface and down the profile was irregular in all the patches. The pasture area covered by Br^– in the sheep patches was 0.04–0.06 m² and Br^– was detected to a depth of 150 mm. Cattle patches were significantly larger covering a surface area of 0.38–0.42 m² and penetrating to a depth of 400 mm. The rapid downward movement of urine occurred through macropore flow but even so, over half of the applied Br^– was detected in the 0–50 mm soil layer in both sheep and cattle patches. Due to the larger volume of urine added to the cattle patches (2000 mL for cattle and 200 mL for sheep) the effective application rate was about 5 L m^–2 compared with 4 L m^–2 for sheep. Concentrations of extractable mineral N and ionic concentrations in soil solution were higher in cattle than sheep patches particularly near the soil surface. In both sheep and cattle patches, urea was rapidly hydrolysed to NH ₄ ⁺ and nitrification occurred between 14 and 29 days after urine application. Initially the major anions and cations in the soil solution were HCO ₃ ^– , SO ₄ ⁼ , Cl^–, NH ₄ ⁺ , Mg⁺⁺, K⁺ and Na⁺, which were derived from the urine application. Ionic concentrations in the soil solution decreased appreciably over time due to plant uptake and possibly some leaching. As nitrification proceeded, NO ₃ ^– became the dominant anion in soil solution and the major accompanying cation was Ca⁺⁺. The fate of¹⁵N-labelled urine-urea was followed during a 5 month period beginning in late autumn. Greater leaching losses of NO ₃ ^– occurred below cattle patches (equivalent to 60 kg N ha^–1 below 300 mm and 37 kg N ha^–1 below 600 mm) compared with sheep patches (10 kg N ha^–1 below 300 mm and 1 kg N ha^– below 600 mm). While 6% of the applied¹⁵N was leached the amount of N leached was equivalent to 11% of the applied urine-N in cattle patches. This suggests that there was significant immobilsation-mineralisation turnover in urine patch soil with the release of mineral N from native soil organic matter. In both sheep and cattle patches 60% of the¹⁵N was accounted for in plant uptake, remaining in the soil and leaching. About 40% of the applied N was therefore lost through gaseous emission.     

Denitrification and N2O emissions from a UK pasture soil following the early spring application of cattle slurry and mineral fertiliser

Total denitrification and nitrous oxide (N₂O) losses were measured from three contrasting dairy management systems representing good commercial practice (system 1), production maintained but with reduced N losses (system 2); and nitrate leaching less than 50 mg L^-1 but with reduced production (system 3). Measurements were made following mineral fertiliser application and from two plot experiments where four treatments were applied: control, NH₄NO₃ at 60 kg N ha^-1, cattle slurry applied to the surface (equivalent to 45 kg N ha^-1), and cattle slurry injected. Despite low soil temperatures (<6 °C) and low rainfall (<3 mm), total denitrification and N₂O losses peaked at 56 and 16 g N ha^-1 d^-1, respectively. Total denitrification losses decreased: system 1 system 2 > system 3, whereas N₂O losses decreased: system 2 > system 3 > system 1. Total denitrification losses tended to decrease with decreasing fertiliser application rate, whereas fertiliser application rate was not the sole determinant of the N₂O loss. The system 3 field was injected with cattle slurry for 2 yr, system 2 received some slurry by injection and system 1 received slurry to the surface. Thus, the amount, timing and method of previous cattle slurry application was important in determining the loss following subsequent fertiliser application. For the plot experiments, total denitrification and N₂O losses decreased in the order: slurry injected > mineral fertiliser > slurry applied to the surface > control for 5 days following application. However, 16 and 19 days after application, N₂O losses above the control were measured from plots that had received cattle slurry. It was inferred that the application of cattle slurry to the pasture soil stimulated greater N₂O production and increased losses over a longer time period compared with mineral fertiliser additions.     

The change of soil carbon stocks and fine root dynamics after land use change from a native pasture to a pine plantation

A published meta-analysis of worldwide data showed soil carbon decreasing following land use change from pasture to conifer plantation. A paired site (a native pasture with Themeda triandra dominant, and an adjacent Pinus radiata plantation planted onto the pasture 16 years ago) was set up as a case study to assess the soil carbon reduction and the possible reason for the reduction under pine, including the change in fine root (diameter <2 mm) dynamics (production and mortality). Soil analysis confirmed that soil carbon and nitrogen stocks to 100 cm under the plantation were significantly less than under the pasture by 20 and 15%, respectively. A 36% greater mass of fine root was found in the soil under the pasture than under the plantation and the length of fine root was about nine times greater in the pasture. Much less fine root length was produced and roots died more slowly under the plantation than under the pasture based on observations of fine root dynamics in minirhizotrons. The annual inputs of fine root litter to the top 100 cm soil, estimated from soil coring and minirhizotron observations, were 6.3 Mg dry matter ha⁻¹ year⁻¹ (containing 2.7 Mg C and 38.9 kg N) under the plantation, and 9.7 Mg ha⁻¹ year⁻¹ (containing 3.6 Mg C and 81.4 kg N) under the pasture. The reduced amount of carbon, following afforestation of the pasture, in each depth-layer of the soil profile correlated with the lower length of dead fine roots in the layer under the plantation compared with the pasture. This correlation was consistent with the hypothesis that the soil carbon reduction after land use change from pasture to conifer plantation might be related to change of fine root dynamics, at least in part.     

Modelling the change in soil organic C and N and the mineralization of N from soil in response to applications of slurry manure

A computer simulation model of the turnover of organic matter in soil was adapted to simulate the change in soil organic C and N contents of soil during several years following annual additions of farm slurry to maize fields. The model proved successful in estimating the build-up of both C and N in soil and the leaching of N to ground-water in response to applications of slurry ranging from 50 to 300 tons per hectare per year. The model was then used to estimate the build-up of organic matter in soil under crops of fodder maize that were grown using the excess of manure produced during the last 20 years in the Netherlands. The build-up of organic matter from these applications was estimated to lead to about 70 kg extra nitrogen mineralized ha^-1 yr^-1. As a result of legislation manure applications have decreased and are expected to decrease further in the immediate future. Calculations suggest that after 10 years of manure applied at rates no longer exceeding the amount needed to replace the phosphorus removed by crops, the extra mineralization of N will still be between 45 and 60 kg ha^-1 yr^-1. If manure applications cease altogether then the extra mineralization will be about 25–30 kg N ha^-1 yr^-1.     

Land-use change effects on soil C and N transformations in soils of high N status: comparisons under indigenous forest, pasture and pine plantation

Globally, land-use change is occurring rapidly, and impacts on biogeochemical cycling may be influenced by previous land uses. We examined differences in soil C and N cycling during long-term laboratory incubations for the following land-use sequence: indigenous forest (soil age = 1800 yr); 70-year-old pasture planted after forest clearance; 22-year-old pine (Pinus radiata) planted into pasture. No N fertilizer had been applied but the pasture contained N-fixing legumes. The sites were adjacent and received 3–6 kg ha^–1 yr^–1volcanic N in rain; NO₃ ^–-N leaching losses to streamwater were 5–21 kg ha^–1 yr^–1, and followed the order forest < pasture = pine. Soil C concentration in 0–10 cm mineral soil followed the order: pasture > pine = forest, and total N: pasture > pine > forest. Nitrogen mineralization followed the order: pasture > pine > forest for mineral soil, and was weakly related to C mineralization. Based on radiocarbon data, the indigenous forest 0–10 cm soil contained more pre-bomb C than the other soils, partly as a result of microbial processing of recent C in the surface litter layer. Heterotrophic activity appeared to be somewhat N limited in the indigenous forest soil, and gross nitrification was delayed. In contrast, the pasture soil was rich in labile N arising from N fixation by clover, and net nitrification occurred readily. Gross N cycling rates in the pine mineral soil (per unit N) were similar to those under pasture, reflecting the legacy of N inputs by the previous pasture. Change in land use from indigenous forest to pasture and pine resulted in increased gross nitrification, net nitrification and thence leaching of NO₃ ^–-N.