A. Parshotam

Is soil acidification the cause of biochemical responses when soils are amended with heavy metal salts

Three New Zealand soils of contrasting texture, organic matter content and cation exchange capacity (CEC) were amended with solutions of the nitrate salts of Cd[II], Cr[III], Cu[II], Ni[II], Pb[II] and Zn[II], spanning the concentration range, 0–100 mmol heavy metal kg−1 soil. Additional treatment sets comprising: (1), the same range of Ca(NO3)2 concentrations to account for osmotic effects, and (2), the same range of NO3− concentrations, comprising NaNO3 acidified with HNO3, to account for the acidifying effects of metal salt amendment of the soils, were also included. Samples were assayed for phosphatase and sulphatase enzyme activities, and for basal respiration and substrate-induced respiration (SIR), approximately 1 week after amendment. Metal amendment resulted in considerable acidification of all three soils, with the metals which hydrolyse most (Cr, Cu and Pb) having the greatest effect, and the coarsest textured soil being the most affected. Phosphatase activity declined by up to 70% as a result of metal amendment in the finer-textured soils, but acid amendment had little or no effect. In the coarse-textured soil, neither acid nor most metals inhibited phosphatase activity until pH fell to below 4. In contrast, sulphatase activity was strongly inhibited by acid and by all metal amendments, including Ca, in all three soils, indicating that acidification was the dominant effect. Soil respiration declined markedly, and approximately equally, at low metal and acid amendment concentrations in the lighter-textured soils. Activity then stabilised with little further decline occurring until high metal amendment reduced soil pH to 4–4.5. In the coarse-textured soil, metal amendment resulted in a similar sharp initial decline of respiration, but acid had a proportionately lesser effect. In all three soils, metal amendment (except Ca) generally resulted in greater inhibition of SIR response than did acid amendment, although the acid effect was substantial. Cadmium and, especially, Ni were the most inhibitory metals. These results showed that, for all biochemical activities assayed except phosphatase, a considerable proportion of the inhibition occurring when soils are amended with heavy metal salts can be attributed to acidification and not a direct metal effect. In the extreme case, sulphatase activity, acidification may account for most or all of the inhibitory effect.

14C-labelled glucose turnover in New Zealand soils

The influence of soil mineralogy, as well as texture, on organic-C turnover was determined with 14C-labelled glucose. Samples of 16 soils from major mineralogical classes of New Zealand pastures and providing a range of organic C, clay contents and surface area, were incubated with 14C-labelled glucose for 35 d. The amounts of 12CO2 and 14CO2 evolved during incubation were monitored and the residual 14C concentrations determined. Periodically, the samples were removed and microbial biomass 12C and 14C determined using the fumigation-extraction technique. System mean residence times (MRTs) were obtained by three independent methods: (i) a compartmental model using 14C microbial biomass data, (ii) a non-compartmental model using 14C microbial biomass data and (iii) a biexponential equation as an empirical equation from residual 14C data. The effect of soil characteristics on MRTs was compared. The 14CO2 respired, after 35 d incubation, accounted for 51 to 66% of the glucose 14C input to these soils. The soils differed significantly in their amounts of 14CO2 evolution and in the proportions of labelled 14C in the biomass. The extent of mineralization of 14C-labelled glucose was influenced by soil clay content and clay surface area. Soils of low clay content (3–12%) had high biophysical quotients (respired: residual 14C); the highest (1.93) was in the soil with least clay (3%) and lowest mineral surface area, suggesting that clay is effective in C stabilization immediately after substrate assimilation. A biexponential model was found to be suitable for describing changes in the residual 14C and microbial biomass 14C during the 35 d glucose decomposition for most of the soils. MRTs for microbial biomass 14C were correlated with clay content (P<0.001), surface area estimated by para-nitrophenol (pNP) (P<0.003) and pH (P<0.01). Our results also showed that the MRTs of microbially assimilated 14C are similar despite differences in the chemical nature of the applied 14C-labelled substrate. However, the MRT for humus 14C differed with the chemical nature of the applied substrate. Clay and surface area played a major role in controlling the decomposition of added substrate through the stabilization and protection of the microbial biomass.

A simple kinetic approach to derive the ecological dose value, ED50, for the assessment of Cr(VI) toxicity to soil biological properties

Three New Zealand soils of contrasting texture, organic matter content and cation exchange capacity (CEC) were amended with K2Cr2O7 solutions, spanning two concentration ranges, 0–5 μmol Cr(VI) g−1 soil and 0–50 μmol Cr(VI) g−1 soil. Samples were assayed for phosphatase, sulphatase and urease enzyme activities, and for basal respiration, microbial biomass C, dimethyl sulphoxide(DMSO)-reducing activity and denitrification, 3 and 60 d after amendment. Extractability of Cr(VI) from similarly amended samples was measured from 0 to 100 d. Cr(VI) proved to be strongly inhibiting of most of the biological properties and in most instances inhibition was explained by one or both of two simple Michaelis-Menten kinetic models. The first of these (Model 1) simulated fully competitive kinetics and the second (Model 2) simulated partially competitive kinetics. A single inhibition constant, similar to ED50 as conceptualized in previous studies, could usually be calculated for each property in each soil. The properties could be ranked in the following order of decreasing sensitivity to Cr(VI): denitrification > DMSO-reduction > sulphatase activity ≈ biomass C > phosphatase activity > urease activity > respiration. For the most sensitive property, denitrification, ED50 values range from 63 to 730 nmol Cr g−1 soil. Soil mineral surface area, organic matter content and CEC influenced the sensitivity of properties between soils. Although the extent of inhibition often diminished with time, the differences were generally much smaller than the observed decline in extractability of Cr(VI), indicating that a persistent, long-term inhibition, outlasting the Cr(VI) itself, had occurred.

Carbon turnover in a range of allophanic soils amended with 14C-labelled glucose

Abstract The influence of soil allophane (a short-range-order mineral) content on organic-C turnover was determined with 14 C-labelled glucose. Samples from four soils, providing a range of allophane, organic C, clay contents, and some other characteristics, were incubated with 14 C-labelled glucose for 28 days. During incubation, microbial biomass 12 C and 14 C were determined using the fumigation-extraction technique. The amounts of 12 CO 2 and 14 CO 2 evolved during incubation were also monitored, and residual 14 C concentrations determined. Biomass 14 C was highest in the soil with the highest allophane content (13%) and least in the soil with least allophane content ( 14 CO 2 production from the [ 14 C]glucose was highest (63%) in the soil with least allophane content and lowest (54%) in the soil with the most allophane. It was concluded, from first-order decay rate constants for residual 14 C and exponential decay rate constants for biomass 14 C, that allophane retards the turnover rates of 14 C derived from added glucose by stabilization of microbial biomass, and also by protection of microbial products. During a 28 day incubation, ca 0.8% more C was diverted from respired CO 2 to new biomass with each 1% increase in allophane content. For allophanic soils, inclusion of mineral surface area rather than clay content should provide a better quantification of the organic matter turnover rate.

Simple kinetic approach to determine the toxicity of AS[V] to soil biological properties

Three New Zealand soils of contrasting texture, organic matter content and CEC were amended with Na2HAsO4·7H2O solutions, spanning the concentration range, 0–50 μmol As[V] g−1 soil. Samples were assayed for phosphatase, sulphatase and urease enzyme activities and for basal respiration, microbial biomass C (by substrate-induced respiration, SIR), dimethyl sulphoxide (DMSO)-reducing activity and denitrification, 3 and 60 d after amendment. Only phosphatase, sulphatase and DMSO-reducing activities were consistently inhibited by As[V], the remaining properties were generally unaffected or were stimulated. When inhibition occurred, it could in most instances be explained by one or both of two simple Michaelis Menten kinetic models. The first of these (model 1) described fully competitive kinetics and the second (model 2) described partially competitive kinetics. A single inhibition constant, similar to ED50 (ecological dose) as conceptualised in previous studies, could be calculated. In comparison with heavy metals, As[V] was not a potent inhibitor of soil biochemical properties, with ED50 values ranging from 2.18–556 μmol As g−1 soil (0.163–41.7 g kg−1). Generally, phosphatase was the most sensitive property, probably due to the structural similarity of phosphate and arsenate. Basal respiration and denitrification were the most activated properties, the former increasing linearly with increasing As[V] concentration. Soil textural characteristics influenced the sensitivity of properties between the different soils; the coarsely textured sandy soil was both the most biochemically sensitive to and the least sorptive of As[V]. For one soil only there was a consistent effect of time since amendment, with diminished inhibition or enhanced activation at 60 d compared with 3 d.

Carbon turnover in two soils with contrasting mineralogy under long-term maize and pasture

Total organic carbon (C) and natural 13C abundance were measured in adjacent allophanic soils (Andisols) and non-allophanic soils (Inceptisols) under maize (Zea mays L.) and ryegrass pasture (Lolium perenne L.) to assess the C turnover rate in soils of contrasting mineralogy and specific surface area. The allophanic soil contained more total C than the non-allophanic soil (139 v. 101 t Csha in the upper 0–35 cm) but neither soil showed a significant difference in C content between pasture and maize, provided maize residue was retained and incorporated. The gross annual inputs under maize and pasture were both estimated to be about 9 t Csha, consistent with the similar soil total C contents. Since the soil total C content did not appear to change with time, the gross C mineralisation was about 9 t Csha each year. The proportion of old pasture C remaining in the soil after maize cropping for 25 years was about 78p in the allophanic soils and about 69p in the non-allophanic soils, reflecting the influence of both Al and allophane, with its high specific surface area, on the retention of old C. The maize C retained in 25 years was similar in both the allophanic soil and the non-allophanic soil (29 tsha). Therefore, the higher total C content of the allophanic soil would appear to arise from stabilisation of old pasture and forest C by Al and allophane. clover, grass, New Zealand soils.

Processes influencing soil carbon storage following afforestation of pasture with Pinus radiata at different stocking densities in New Zealand

Since 1992, afforestation with Pinus radiata D. Don in New Zealand has led to the establishment of over 600 000 ha of new plantation forests, about 85% of which are on fertile pastures used previously for grazing sheep and cattle. While this leads to rapid accumulation of carbon (C) in vegetation, the effects of afforestation on soil C are poorly understood. We examined key soil C cycling processes at the (former) Tikitere agroforestry experimental site near Rotorua, New Zealand. In 1973, replicated stands of P. radiata (100 and 400 stems/ha) were established on pastures, while replicated pasture plots were maintained throughout the first 26-year rotation. In 1996, soil C and microbial biomass C in 0-0.10 m depth soil, in situ soil respiration and net N mineralisation, and soil temperature were lower in the forest than in the pasture, and tended to decline with increasing tree-stocking density. In the 400 stems/ha stands, mineral soil C (0-0.50 m depth) was lower than in the pasture (104 and 126 Mg C/ha, respectively; P < 0.01). Carbon accumulation in the forest floor during the first rotation of these forest stands was 12 Mg C/ha. Using the Rothamsted soil C model (Roth-C), we examined how changes in plant C inputs following afforestation might lead to changes in soil C content to 0.30 m depth. Steady-state pasture inputs of 9.0 Mg C/ha.year were estimated using Roth-C; these C inputs were assumed to decrease linearly during the first 12 years following tree establishment (until canopy closure). Below-ground C inputs in the forest were estimated using steady-state relationships between litterfall and soil respiration; these inputs were assumed to increase linearly between years 1 and 12, after which they remained constant at 1.53 Mg C/ha.year until harvest. Measured changes in soil C (0−0.30 m) during the first rotation, in conjunction with the below-ground inputs, were used to estimate above-ground inputs (as a proportion of total litterfall (3.81 Mg C/ha.year)) to the soil. Our results suggest 10% of litterfall C over one rotation actually entered the mineral soil. Using these results and estimates of additional C inputs to the soil from harvest slash and weeds following harvest, we found mineral-soil C stocks would continue to decline during second and third rotations of P. radiata; the magnitude of this decline depended in part on how much slash enters the mineral soil matrix. We confirmed our modelling approach by simulating soil C changes to within 8% over 19 years following afforestation of pasture at another previously studied site, Purukohukohu. Whether afforestation leads to an increase or decrease in mineral-soil C may depend on previous pasture management; in highly productive pastures, high C inputs to the soil may maintain soil C at levels that cannot be sustained when trees are planted onto these grasslands.

Plant effects on soil carbon storage and turnover in a montane beech (Nothofagus) forest and adjacent tussock grassland in New Zealand

Land cover is a critical factor that influences, and is influenced by, atmospheric chemistry and potential climate changes. As considerable uncertainty exists about the effects of differences in land cover on below-ground carbon (C) storage, we have compared soil C contents and turnover at adjacent, unmanaged, indigenous forest (Nothofagus solandri var. cliffortiodes) and grassland (Chionochloa pallens) sites near the timberline in the same climo-edaphic environment in Craigieburn Forest Park, Canterbury, New Zealand. Total soil profile C was 13% higher in the grassland than in the forest (19.9 v. 16.7 kg/m2 ), and based on bomb 14C measurements, the differences mainly resulted from more recalcitrant soil C in the grassland (5.3 v. 3.0 kg/m2 ). Estimated annual net primary production was about 0.4 kg C/m2 for the forest and 0.5 kg C/m2 for the grassland; estimated annual root production was about 0.2 and 0.4 kg C/m2 , respectively. In situ soil surface CO2 -C production was similar in the grassland and the forest. The accumulation of recalcitrant soil C was unrelated to differences in mineral weathering or soil texture, but was apparently enhanced by greater soil water retention in the grassland ecosystem. Thus, contrary to model (ROTHC) predictions, this soil C fraction could be expected to respond to the effects of climate change on precipitation patterns. Overall, our results suggest that the different patterns of soil C accumulation in these ecosystems have resulted from differences in plant C inputs, soil aluminium, and soil physical characteristics, rather than from differences in soil mineral weathering or texture.

The Rothamsted soil-carbon turnover model — discrete to continuous form

Abstract This paper examines the Rothamsted soil-carbon turnover model. This model and other soil carbon turnover models are increasingly being used in climate change and land use studies. The formulation of this model is as a discrete sums-of-exponentials. This discrete form may be converted to a continuous form which can be shown to be an asymptotic approximation of a simple linear first-order system of differential equations where the exponential decay rate constants in the sums-of-exponentials form approximate the first-order rate transfer coefficients in the differential equation form. There are many standard analytical results that can be obtained from this linear system of differential equations that could prove to be very useful in modelling soil-carbon turnover.

Application of the Rothamsted carbon turnover model to soils in degraded semi-arid land in New Zealand

The Rothamsted soil-carbon turnover model was used to determine rates of change of organic carbon in soils of a degraded semi-arid land system of New Zealand. Estimates of annual inputs, net primary production (NPP), and recovery time to raise carbon from a degraded state to a sustainable level of production were obtained. The annual inputs and hence the NPP were estimated by running the Rothamsted soil-carbon turnover model in reverse. Inputs included the organic C content of the soil, clay content, monthly temperature and rainfall and the rate of decay of annual inputs. An estimate of the biologically inert organic matter fraction was obtained from the radiocarbon content of soil organic matter. The soil recovery time was estimated by determining NPP for a relatively undegraded soil and and applying carbon at this rate to a degraded soil. The predicted total annual plant residue inputs were 1.81 t ha−1 y−1 and the predicted inert organic matter content from radiocarbon data is 5.12 t ha−1. The estimated time needed to raise carbon from a degraded state to the level of an undegraded site is 48 years. This is most likely an underestimate, because in initial years the vegetation will not be capable of adding the same annual carbon input as the undegraded site. The refinement of this estimate of recovery time will require data on the rate of increase in net primary productivity with revegetation.