Christopher J. Weston

In situ studies of nitrogen mineralization and uptake in forest soils; some comments on methodology

Abstract Aspects of the methodology of and interpretation of results from, in situ studies of N-mineralization are discussed with reference to data collected from 17 eucalypt forests in south-eastern Australia during a 5-year period. Results suggest that: (i) it is possible to maintain moisture of soils contained within corers at levels not significantly different from those of the surrounding soil; (ii) inorganic-N is not produced linearly over time under field conditions, nor should we expect it to be; (iii) mineralization rates are affected by all in situ methods. In each of the forests examined, the average rate of net N-mineralization decreased as the period of containment increased; (iv) shorter periods of containment (e.g. 1–2 weeks) reduce artifacts due to containment and are therefore preferable to longer periods (e.g. 4–8 weeks); and (v) longer periods of containment cannot substitute for increased replication of sampling. Further, when annual or seasonal rates of N-mineralization and uptake are to be calculated, it is desirable to increase replication of the bulk soil sampling for the first and last sampling dates.

Temporal dynamics of the carbon isotope composition in a Pinus sylvestris stand: from newly assimilated organic carbon to respired carbon dioxide

The 13C isotopic signature (C stable isotope ratio; δ13C) of CO2 respired from forest ecosystems and their particular compartments are known to be influenced by temporal changes in environmental conditions affecting C isotope fractionation during photosynthesis. Whereas most studies have assessed temporal variation in δ13C of ecosystem-respired CO2 on a day-to-day scale, not much information is available on its diel dynamics. We investigated environmental and physiological controls over potential temporal changes in δ13C of respired CO2 by following the short-term dynamics of the 13C signature from newly assimilated organic matter pools in the needles, via phloem-transported organic matter in twigs and trunks, to trunk-, soil- and ecosystem-respired CO2. We found a strong 24-h periodicity in δ13C of organic matter in leaf and twig phloem sap, which was strongly dampened as carbohydrates were transported down the trunk. Periodicity reappeared in the δ13C of trunk-respired CO2, which seemed to originate from apparent respiratory fractionation rather than from changes in δ13C of the organic substrate. The diel patterns of δ13C in soil-respired CO2 are partly explained by soil temperature and moisture and are probably due to changes in the relative contribution of heterotrophic and autotrophic CO2 fluxes to total soil efflux in response to environmental conditions. Our study shows that direct relations between δ13C of recent assimilates and respired CO2 may not be present on a diel time scale, and other factors lead to short-term variations in δ13C of ecosystem-emitted CO2. On the one hand, these variations complicate ecosystem CO2 flux partitioning, but on the other hand they provide new insights into metabolic processes underlying respiratory CO2 emission.

δ13C of organic matter transported from the leaves to the roots in Eucalyptus delegatensis: short-term variations and relation to respired CO2

Post-photosynthetic carbon isotope fractionation might alter the isotopic signal imprinted on organic matter (OM) during primary carbon fixation by Rubisco. To characterise the influence of post-photosynthetic processes, we investigated the effect of starch storage and remobilisation on the stable carbon isotope signature (δ13C) of different carbon pools in the Eucalyptus delegatensis R. T. Baker leaf and the potential carbon isotope fractionation associated with phloem transport and respiration. Twig phloem exudate and leaf water-soluble OM showed diel variations in δ13C of up to 2.5 and 2‰, respectively, with 13C enrichment during the night and depletion during the day. Damped diel variation was also evident in bulk lipids of the leaf and in the leaf wax fraction. δ13C of nocturnal phloem exudate OM corresponded with the δ13C of carbon released from starch. There was no change in δ13C of phloem carbon along the trunk. CO2 emitted from trunks and roots was 13C enriched compared with the potential organic substrate, and depleted compared with soil-emitted CO2. The results are consistent with transitory starch accumulation and remobilisation governing the diel rhythm of δ13C in phloem-transported OM and fragmentation fractionation occurring during respiration. When using δ13C of OM or CO2 for assessing ecosystem processes or plant reactions towards environmental constraints, post-photosynthetic discrimination should be considered.

Effects of fire and harvesting on nitrogen transformations and ionic mobility in soils of Eucalyptus regnans forests of south-eastern Australia

SummaryEffects of fire and forest harvesting on inorganic-N in the soil, on net N-mineralization, and on the leaching of NOinf3sup--N and metallic cations were measured in forests of Eucalyptus regnans following a severe wildfire in 1983. E. regnans regenerates profusely by seed after fire, and this study compared unburnt forest with forests burnt at varying intensities (surface fire and crown fire), and with logged and burnt forest (slash fire). Total inorganic-N in soil (0–5 cm) increased with increasing fire intensity to a maximum of 158 μg g-1 in the slash fire plot (compared with 51 μg g-1 in the unburnt forest) over the first 205 days after fire. Total inorganic-N returned to a concentration equal to that in the unburnt forest after 485 days at the slash fire plot, and after only 205 days at the surface fire plot. Studies of net mineralization in situ and of NOinf3sup--N in soil solution support the hypothesis that inorganic-N was immobilized in all of the burnt forests; microbial immobilization after fire is identified as a key process in N-conservation, limiting the substrate available for nitrification and thereby limiting the loss of N from the system by leaching. The concentrations of NOinf3sup--N and metallic cations in soil solution increased with increasing fire intensity. For the first 318 days after the fire, [NOinf3sup--N] in soil solution at 10 cm averaged 0.6 μg ml-1 in the unburnt forest, 9.7 mg l-1 in the surface fire plot, 26 mg l-1 in the crown fire plot, and 70 mg l-1 in the slash fire plot. The concentration of metallic cations in soil solution was significantly correlated with [NOinf3sup--N], the observed order of mobility being Ca2+>Mg2+>K+>Na+. Processes which limit the production and persistence of NOinf3sup--N in soil solution following disturbance will significantly reduce nutrient losses or redistribution.

The response of growth and foliar nutrients to fertilizers in young Eucalyptus globulus (Labill.) plantations in Gippsland, southeastern Australia

Abstract Fertilizer trials, which included rates of application of up to 400 kg ha−1 N, 200 kg ha−1 and 200 kg ha−1 K, were established in experimental plantations of E. globulus at three sites in Gippsland, southeastern Australia. These sites cover a range of annual rainfall and soil fertility, from 620 mm on a deep infertile sand, to 1000 mm on a rich gradational clay loam. Early growth at all sites was significantly increased by the addition of fertilizers and by age 4 years was consistently best at the highest combined rates of N and P. Foliar N and P concentrations were significantly increased at all sites by combined additions of N and P at Age 1, but were insensitive to treatment at Ages 2 and 4. Relationships between foliar nutrient concentrations and growth were site dependent. Correlations between foliar nutrients and growth in the same year were strongest at Age 1. Foliar nutrients at Age 1 were also strongly associated with growth at later ages at all sites. The best growth was associated with foliar concentrations at Age 1 of about 2.5% N and 0.23% P. Additions of P, alone or with N, consistently decreased foliar N P ratios at Age 1 to between 11 and 12 whereas N and P additions resulted in foliar N P ratios of 15 to 16 at Age 4. These ratios indicate a greater requirement for P than N in the first year suggesting that the N P ratio of fertilizers should be 1:1 within the first year, increasing to 2:1 in later applications. General recommendations for fertilizer addition in the routine establishment of E. globulus are presented. Mean tree volumes at Age 4 ranged from 0.014 to 0.019 m3 in control treatments and from 0.031 to 0.055 m3 at the highest rate of fertilizer addition. Growth responses to fertilizers were greatest, in both absolute and relative terms, at the most fertile site and appeared to be constrained by unfavourable soil texture and limited water availability at the other sites. Because the responses of E. globulus are site specific, detailed assessments of fertilizer requirements on soil types other than those described here require the establishment of further trials. A schedule for the establishment and monitoring of such trials is proposed.

Clearfelling and burning effects on nitrogen mineralization and leaching in soils of old-age Eucalyptus regnans forests

An argument against clearfelling and burning operations in forests is that nutrient reserves may be diminshed, leading to productivity decline over successive rotations. Nitrogen is of primary concern as it is readily volatilised and may be leached and thus the retention of nitrogen is a key recovery process following perturbation. In this study we measured N mineralization in situ and nitrogen concentrations in soil water from an old-age Eucalyptus regnans forest (about 250 years old) and from clearfelled forest in which treatment areas of unburnt ground, burnt ground and burnt ground maintained free from regrowth were established. Total inorganic N in the soil (0–5 cm) increased to a maximum of 168 μg g−1 of dry soil in clearfelled forest following burning, compared with 33 μg g−1 of dry soil in undisturbed forest. Increased total inorganic N in the soil returned to a concentration equal to that in undisturbed forest most rapidly in the clearfelled unburnt forest (6–9 months) and persisted for the longest amount of time in the most severely disturbed site (clearfelled burnt + herbicide-treated forest; 15–18 months). Net annual N mineralization in undisturbed forest soils (0–5 cm) averaged 74.9 μg g−1 of dry soil over the two years of the study. In contrast, annual average N mineralization was negative in two of the three clearfelled sites with 148.3 μg g−1 of dry soil of N immobilized in clearfelled and burnt forest. The concentration of NO3− in soil water increased with increasing forest disturbance. Over the first 260 days following clearfelling NO3−-N concentrations in soil water at 10 cm depth averaged 2.6 mg l−1 in undisturbed forest, 8.5 mg l−1 in clearfelled unburnt forest, 24.2 mg l−1 in clearfelled burnt forest, and 60.3 mg l−1 in clearfelled burnt + herbicide treated forest. Studies of net N mineralization in situ, and of NO3− in soil water, support the hypothesis that inorganic N was immobilized in all disturbed forests. Immobilization of N by soil micro-organisms is alone not sufficient to limit nitrification and NO3− leaching in disturbed E. regnans forests. Rapid uptake of N by regrowing vegetation is essential in reducing the availability of substrate for nitrification (NH4+) as well as in reducing NO3− concentrations in soil water. Clearfelling of the E. regnans catchment in this study did not significantly increase streamwater NO3− concentrations and demonstrates the resilience of E. regnans forests to leaching losses of N following destructive disturbance.

The effects of fertilizers on early growth and foliar nutrient concentrations of three plantation eucalypts on high quality sites in Gippsland, southeastern Australia

Abstract Fertilizer trials of a standard design were established in experimental plantations of Eucalyptus regnans, Eucalyptus nitens and Eucalyptus globulus on sites of high potential productivity in Gippsland, southeastern Australia. The design involved factorial combinations of three rates of N (up to 400 kg ha −1 elemental) and four rates of P (up to 200 kg ha −1 ) with four additional treatments including K and trace elements. Treatments were complete after up to four applications at 2, 9, 14 and 26 months post-planting. P additions significantly increased foliar P concentrations and growth of E. regnans to 45 months but poor survival and competition from woody weeds limited growth responses to treatments. Foliar concentrations of N and P in E. nitens and E. globulus at age 1 year were significantly increased by N and P additions and were positively correlated with both early and later growth. Added P significantly decreased the foliar N to P ratios of all species at age 1 year, indicating that P additions, in particular, were required for good early growth. Combined additions of N and P significantly increased growth of E. nitens in the first year but growth responses to further additions of fertilizer were minimal. In contrast, growth of E. globulus increased in response to all fertilizer additions, leading to the greatest mean volumes at the highest combined rates of N and P. Relative gains in productivity were therefore greater in E. globulus (m.a.i. range at 71 months: 10–24 m 3 ha −1 year −1 ) than E. nitens (16–33 m 3 ha −1 year −1 ). There were no additional growth responses of any species to added K or trace elements. However, concentrations of K in E. regnans foliage at age 1 year were increased by K additions and were correlated with later growth. Low concentrations of Mg in E. nitens foliage suggest a probable response to Mg additions. General fertilizer prescriptions for the establishment phase of eucalypt plantations on sites of high quality are presented. Refinement of these prescriptions will rely on a better understanding of the nutritional requirements of individual species.

Fuel reduction burning mitigates wildfire effects on forest carbon and greenhouse gas emission

A high-intensity wildfire burnt through a dry Eucalyptus forest in south-eastern Australia that had been fuel reduced with fire 3 months prior, presenting a unique opportunity to measure the effects of fuel reduction (FR) on forest carbon and greenhouse gas (GHG) emissions from wildfires at the start of the fuel accumulation cycle. Less than 3% of total forest carbon to 30-cm soil depth was transferred to the atmosphere in FR burning; the subsequent wildfire transferred a further 6% to the atmosphere. There was a 9% loss in carbon for the FR–wildfire sequence. In nearby forest, last burnt 25 years previously, the wildfire burning transferred 16% of forest carbon to the atmosphere and was characterised by more complete combustion of all fuels and less surface charcoal deposition, compared with fuel-reduced forest. Compared to the fuel-reduced forests, release of non-CO2 GHG doubled following wildfire in long-unburnt forest. Although this is the maximum emission mitigation likely within a planned burning cycle, it suggests a significant potential for FR burns to mitigate GHG emissions in forests at high risk from wildfires.

Visual assessments of fuel loads are poorly related to destructively sampled fuel loads in eucalypt forests

Fire managers around the world commonly use visual assessment of forest fuels to aid prediction of fire behaviour and plan for hazard reduction burning. In Australia, fuel hazard assessment guides also allow conversion of visual assessments to indicative fuel loads, which is essential for some rate of spread models and calculation of fireline intensity or emissions. The strength of correlation between fuel hazard and destructively sampled (directly measured) fuel load was tested using a comprehensive dataset of >500 points from across a range of eucalypt forests in Australia. Overall, there was poor correlation between the assigned fuel hazard rating and measured biomass for surface, near-surface and elevated fuel components, with a clear tendency for these systems to under-predict fuel load at low hazard ratings, and over-predict it at high hazard ratings. Visual assessment of surface fuels was not statistically different from a random allocation of hazard level. The considerable overlap in fuel load between hazard ratings at higher ranges suggests the need to reduce the number of hazard classes to provide clearer differentiation of fuel hazard. To accurately assess forest fuel condition, improvements in fuel hazard descriptions and calibration of visual assessment with destructively measured fuels is essential.

Mid-infrared spectroscopy for rapid assessment of soil properties after land use change from pastures to Eucalyptus globulus plantations

There is an increasing demand for rapid and cost effective techniques to accurately measure the effects of land use change on soil properties. This study evaluated the ability of mid-infrared spectroscopy (MIRS) coupled with partial least squares regression (PLSR) to rapidly predict soil properties affected by land use change from agriculture (mainly pasture) to Eucalyptus globulus plantations in south-western Australia. We measured total organic carbon (TOC), total nitrogen (Total N), TOC/Total N (C/N ratio), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), and total phosphorus (Total P). The PLSR calibration models were developed using mid-infrared (MIR) spectra (4000 to 450 cm(-1)) and square root transformed measured soil data (n = 180) from 23 paired pasture and E. globulus plantation sites representing the soils and climate of E. globulus plantation estates in south-western Australia. The calibration models for TOC, Total N, C/N ratio and Total P showed excellent correlations between measured and predicted data with coefficient of determination (R(2)) exceeding 0.91 and minimum root-mean-square error (RMSE) of calibration [TOC (R(2) = 0.95, RMSE = 0.36), Total N (R(2) = 0.96, RMSE = 0.10), C/N ratio (R(2) = 0.92, RMSE = 0.14) and Total P (R(2) = 0.91, RMSE = 0.06)]. The calibration models had reasonable predictions for MBC (R(2) = 0.66, RMSE = 0.07) and MBN (R(2) = 0.63, RMSE = 0.06). The calibrated models were validated using soils from 8 independent paired pasture and E. globulus sites (n = 64). The validated predictions were excellent for TOC (R(2) = 0.92, RMSE = 0.40) and Total N (R(2) = 0.91, RMSE = 0.12), but less so for C/N ratio (R(2) = 0.80, RMSE = 0.35), MBC (R(2) = 0.70, RMSE = 0.08) and Total P (R(2) = 0.75, RMSE = 0.12). The results demonstrate the potential of MIRS-PLSR to rapidly, accurately and simultaneously determine several properties in land use change affected soils.