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Soil nitrogen mineralisation changes rapidly when pine is planted into herbicide-treated pasture--the first two years of growth.

Introduction

Land under pasture in New Zealand has been converted to exotic forest plantations, predominantly Pinus radiata D. Don, at a rate of 20000--100000 ha/year over the last 10 years. This change in land-use affects the properties of the mineral topsoil, and generally during the first 10-20 years of growth of P. radiata the soil pH, C, N, [Ca.sub.exchange], and C and net N mineralisation decrease, and the C 'N ratio, [Na.sub.exchange], and [Mg.sub.exchange] increase (Davis 1994; Giddens et al. 1997; Parfitt et al. 1997; Perrott et al. 1999; Ross et al. 1999). The decrease in total soil N may arise from: uptake of N by P. radiata, shading of legumes, leaching, and/or denitrification of nitrate-N.

The C and N cycles are closely linked in forest soils (Scott et al. 1998), and several studies in New Zealand have demonstrated that soil C can be lost from mineral soil when pine is planted into grassland, since mineralisation of C would exceed inputs of C during early stages of tree growth (Davis 1995; Chen et al. 2000; Davis and Condron 2002). However, mineralisation of C, measured by respiration, is lower in mineral topsoil under pine than pasture (Ross et al. 1999; Chen et al. 2000), and this reflects the lower quantity and quality of the C inputs to the soil under pine (Saggar et al. 2001). Net N mineralisation is also lower under pine than pasture within 9 years of tree planting (Cooper 1986), but it is not known at what stage this change occurs.

To reduce competition from other species for water, nutrients, and light during plantation establishment, some measure of weed control is usually undertaken (Richardson et al. 1993). This can involve 'post-plant' spraying with herbicides, often over a localised area around the young trees (Anon. 1992). The effects of removal of grazing animals, together with the killing of vegetation by herbicides, would be expected to influence soil biochemical processes and nutrient availability in the early stages of afforestation. We therefore established a 30-month trial to investigate in situ changes in soil N mineralisation and N leaching when P. radiata is planted into pasture; accompanying changes in microbial biomass were also determined. In this paper we test the hypotheses that (a) soil net N mineralisation, nitrification, and N leaching would increase immediately around the trees because of lowered C inputs and as a consequent increase in the availability of N, but that (b) these effects, when integrated over the whole plantation, could be small because of the relatively small size of the sprayed areas.

Materials and methods

Site, field experiment and sampling

A field trial was established in winter 1998 on Ashhurst silt loam (Typic Orthic Brown Soil) at Massey University Farm, Palmerston North, at a flat site under continuous long-term pasture (mainly Yorkshire fog, cocksfoot, perennial ryegrass, white clover, and browntop) grazed by sheep. The mean annual rainfall is 995 mm, and the mean annual temperature is 12.9[degrees]C. From winter 1998 to winter 1999, in situ net N mineralisation was monitored at 6 positions in the pasture. In late July 1999, an area 20 by 30 m was fenced off from the remaining pasture; the fenced area only was grazed heavily by sheep for 3 days; 1-year-old P. radiata seedlings (GF 17) were then planted on a 3 by 3 m grid. Twelve mini-plots were established under 12 adjacent pine trees, 12 under the rank grass (ungrazed grass) between the trees, and 12 under the grazed pasture immediately adjacent. Half the mini-plots were randomly selected for monitoring net N mineralisation and half for monitoring soil solution N using suction cup lysimeters. In August 1999, each tree was sprayed once with terbuthylazine (Gardoprim; 20 mL per tree; 600-mm radius) to control ungrazed grass growth, in accord with general forestry practice (Anon. 1992; Richardson et al. 1993). Tree height and root collar diameters of the 12 trees were measured in September 2000 and September 2001. Since the experiment was ongoing, the trees were not cut down and sampled. The standing herbage of rank grass was also measured using 250 by 250 mm quadrats at 5 random positions.

Pasture herbage growth was measured over the periods used for N mineralisation, using 3 randomly located cages; pasture was trimmed to a height of 10 mm before the cages were placed in position. Herbage was sampled after 1-3 months by cutting by hand to 10 mm. It was then dried at 80[degrees]C, weighed, ground, and analysed for N and P content. The uptake of N was calculated from these data.

Suction cup lysimeters were placed at 250 mm depth in the 6 mini-plots to give 6 replicates for each of the pasture, rank grass, and pine treatments. Since the soil was well drained, the lysimeters were sampled after rain had brought the soil water content to field capacity, usually during rain events of >15 mm. The suction cups were left at a suction of 20 kPa for 16 h, and the soil solutions were then removed and immediately frozen for future analysis.

Soil net N mineralisation was monitored with the buried bag technique (Eno 1960; Nadelhoffer et al. 1984), in which soil moisture is retained but [O.sub.2] and C[O.sub.2] can diffuse through the bags during the in situ incubation (Gordon et al. 1987). Briefly, soil cores (100 mm deep, 56 mm diameter), with grass removed, were buried for 1-3 months in sealed polythene bags and covered with a 10-mm soil cap. Cores were taken at random positions within the 6 mini-plots to give 6 replicates for each of the sites under pine, rank grass, and pasture; the cores under the pine trees were taken from random positions in the sprayed zone within 250-500 mm of the tree stem. At the same time, soil at 2 points about 50-100 mm adjacent to the bags was sampled (0-100 mm depth) using a 25-mm-diameter corer, for determination of initial mineral-N (min-N) values. After 1-3 months, the buried bags were removed and the soil was weighed. All these samples were crumbled, and mixed by hand, with any stones removed and weighed, for the subsequent min-N determinations.

Additional soil samples (0-100 mm depth) were taken at random (n = 6) for measurement of total C and N concentration and pH in July each year, and again in September 2002.

In December 2000 total, extractable and microbial C and N were measured on subsamples from the soil cores (0-100 mm depth) used for the initial min-N determinations. The large cores that were buried at this time, after incubation in situ for 70 days, were also subsampled for the same measurements. Both sets of subsamples were first sieved (<5.6 mm) and stored at 4[degrees]C for a few days, with a portion being air-dried and ground (<0.25 mm) for total C and N determinations.

Analytical methods

Results are expressed on the basis of oven-dry (105[degrees]C) weight of single samples of material, unless otherwise stated. All min-N extractions were carried out on the day of soil sampling, and the other biochemical measurements within 1 week of sampling.

Soil pH (in water) and Olsen P were measured according to Blakemore et al. (1987). Soil total C and N were determined by combustion in a Leco FP 2000 analyser and herbage N and P by Kjeldahl digestion (Blakemore et al. 1987). Min-N (N[H.sub.4.sup.+] - N + N[O.sub.3.sup.-] - N) in a 10.0-g sample was extracted with 100 mL 2 M KCl and measured colorimetrically in a Lachat FIA 8000.

Extractable C and N were determined by shaking soil (adjusted to 60% of water-holding capacity) with 0.5 M [K.sub.2] S[O.sub.4] (1/2.5 w/v) for 30 min (Ross et al. 1999); the C extracted was measured with a Shimadzu TOC-5000 total C analyser and N by persulphate oxidation (Ross et al. 1999). Extractable organic N was estimated as the difference between total and extractable min-N. Microbial C and N were determined by fumigation-extraction methods, using a [k.sub.EC]-factor of 0.34 and a [k.sub.EN]-factor of 0.45 (Ross et al. 1999).

Statistical analyses

The significance of differences between the 3 treatments (sites) was determined using ANOVA within the general linear models procedure in MGLH and between 2 treatments by t-tests. The significance of treatment and sampling-time differences in the December 2000 samples (Table 1) was assessed by Repeated Measures analysis; the min-N data were first log-transformed and the nitrate-N percentages analysed as arcsin square root values. Pearson correlations were used to determine relationships between properties.

Results and discussion

Weather data

The weekly rainfall is shown in Fig. la and the mean air temperature and soil water content in Fig. 1b. There was a period of relatively low rainfall early in 1999 and the soil water contents reached very low levels. There were also water deficits in early 2000 and 2001. The winter/spring of 1999 and 2000 were relatively dry, and a water balance model indicated that there were just 40 days when runoff could have occurred.

[FIGURE 1 OMITTED]

Production data

Both pasture production and the N concentration of the pasture herbage varied with season (Fig. 2). Herbage accumulation was 14.5 (s.e.m. 0.4) t/ha in the first year, and its N content was 389 (20) kg/ha. In the second year, values were 13.8 (2.8) t/ha and 336 (70) kg/ha, respectively.

[FIGURE 2 OMITTED]

The tree height was 1.41 (0.19) m after I year, and 2.64 (0.33) m after 2 years, and indicates a rapid rate of growth (Beets and Brownlie 1987; Richardson et al. 1993). The collar diameters were 36 (4) and 74 (5) mm, respectively. The standing herbage of rank grass was 6.5 (0.8) and 6.7 (1.1) t/ha after 1 and 2 years, respectively. In September 2001, the rank grass herbage contained 106 (19) kg N/ha and 12 (3) kg P/ha. Some grass seed had germinated within the area treated with herbicide under the trees by December 2000.

Initial soil properties

In July 1999, the soil (0-100 mm depth) had an average C content of 58 mg/kg, N content of 4.8 mg/kg, C:N ratio of 12.5, Olsen-P of 12 mg/kg, and a pH of 5.7. Soil pH under the pine trees decreased very rapidly to a value of 5.5, and was significantly lower (P < 0.01) than under the other vegetation types by late September 1999. The pH remained about 0.2 units lower under the pines than under the rank grass or pasture until the end of the experiment in September 2001 (see Table 1).

Nitrogen mineralisation under pasture

Rates of net N mineralisation in the soil under pasture varied with season (Fig. 3a) and appeared to be highest in December when temperature and moisture were favourable, and lowest in March when soil moisture was probably limiting (Fig. 1b). In March 1999, N was immobilised during the in situ incubation (Fig. 3a), possibly because of C-rich litter that may have accumulated in the soil cores during the dry summer.

[FIGURE 3 OMITTED]

The total net N mineralisation in soil was 325 kg/ha in the first year, compared with the herbage accumulation N content of 389 (20) kg/ha. In the second year, values were 258 and 336 (70) kg/ha, respectively. Thus, there was reasonable agreement between soil net N mineralisation and N uptake by the pasture herbage on an annual basis. Any discrepancy between the plant uptake of N and net N mineralisation in the soil samples (0-100 mm depth) may have partly arisen from mineralisation in the lower soil layers. Soil net N mineralisation at 100-200 mm depth under pasture can be about 20% that at 0-100 mm depth and could help to explain the discrepancy (Ross et al. 1999; Saggar et al. 2001). We recognise, however, that the methodology does not give precise prediction of net N mineralisation since soil water cannot enter or leave the bags, and roots have been severed.

Nitrogen mineralisation and nitrification in soil under rank grass and trees

In August-September 1999, net soil N mineralisation under the rank grass was particularly high (3.3 kg/ha.day) (data not shown) but had a large s.e.m. (1.5 kg/ha.day), probably because of urine spots resulting from the initial heavy grazing. By October-November, net N mineralisation under the rank grass had decreased to about 0.7 kg/ha.day (Fig. 3b) and was similar to that under pasture (Fig. 3a). In contrast, net N mineralisation in the herbicide-treated areas around the trees was then 1.33 kg/ha.day, and about double that under the pasture or rank grass (P = 0.002) (Fig. 3b). The concentration of nitrate-N in soil solution in the herbicide-treated areas under the trees in October 1999 was very high (81 mg/L) compared with that under pasture (P > 0.001) (Fig. 4a). This is consistent with other reports of nitrate-N increasing in soil and water after the use of herbicide (Likens et al. 1969; Gay et al. 1996; Reynolds et al. 2000). Denitrification can also be enhanced after herbicide treatment of grass (Tenuta and Beauchamp 1996), and there may have been some loss of N by this mechanism.

[FIGURE 4 OMITTED]

Nitrate-N as a percentage of soil mineral-N in soil samples taken at the start of each incubation period is shown in Fig. 5; in September 1999, it was significantly higher (P < 0.001) in the herbicide-treated areas under the trees than under the pasture or rank grass. The mineral-N concentration in the soil under the trees was then 43 (11) mg/kg, which, except for some urine spots, was the highest value recorded in all the mini-plots during the experiment. Since uptake of N by young trees is minimal (Madgewick 1985), there would have been a low demand for mineralised N, thus allowing ammonium-N produced in the soil to be nitrified. The comparatively low pH values in the herbicide-treated areas under the trees are consistent with the generation of protons during nitrification (see Table 1).

[FIGURE 5 OMITTED]

Nitrogen in soil solution and leaching loss

The soil solution ammonium-N concentrations under the rank grass, in the 2 months after heavy grazing from July 1999, were significantly higher than those under the pasture (Fig. 4b). The soil solution nitrate-N concentrations were also higher (P = 0.06), but the standard errors were large (Fig. 4a), probably as a result of the presence of urine patches.

By November 1999, when sheep had been excluded for 3 months and the grass height was about 250 mm, the N concentration in soil solution under rank grass fell to low levels that were not significantly different from those under pasture. The nitrate-N in soil solution under the pine trees was significantly higher (P < 0.05) than under both pasture and rank grass until September 2000 (Fig. 4a). This was consistent with the data in Fig. 3b that showed that net N mineralisation was significantly higher in the soil under the trees than under rank grass during the first year after herbicide treatment. Although net N mineralisation varied with season (Fig. 3), the values under pine were significantly greater than those under rank grass until October 2000 (P = 0.08), when grass was beginning to invade the areas under the trees.

Nitrogen leaching below 25 cm was estimated from the N concentration in soil solution and from a daily water balance. The N leaching losses between July 1999 and September 2000 were 39 kg/ha under the trees, 15 kg/ha under the rank grass, and 3 kg/ha under the pasture. The area under the trees, however, was only 12% of the total land area, giving a contribution of 5 kg/ha under the trees, compared with 13 kg/ha under the rank grass. Most N was leached in July and August 1999 after the heavy grazing. There was little rainfall in the months after herbicide application (Fig. 1a), so the leaching losses were low. The leaching loss between July 1999 and September 2000 for the land under rank grass and trees combined was estimated to be 18 kg N/ha.

Soil total and microbial C and N

In the non-incubated soil sampled about 15 months after tree planting, total C and total N concentrations appeared to be lowest in the soil under pines but the differences were not significant at P = 0.05 (Table 1). Although a slight organic matter gradient may have existed between the 2 adjacent experimental areas, with soil total and microbial C concentrations highest in the pasture, only treatment or spatial effects would have been responsible for any differences between the pine and rank grass treatments. Spatial variability of total C among the 6 replicate samples was high under the trees and rank grass, with the coefficients of variation averaging 7% in the non-incubated samples and 14% in the incubated samples. In spite of this variability, when the data for trees and rank grass alone were analysed, there was a weakly significant difference between soil total C concentrations in the tree and rank grass treatments in both the non-incubated (P = 0.085) and incubated (P = 0.060) samples (Table 1). This decrease in total C concentrations under the trees compared with rank grass was confirmed in September 2002 (P = 0.011); the soil bulk densities did not differ significantly, and the C content was therefore also significantly lower (P = 0.029) under the trees. Lower soil C concentrations under pine than grass have been previously found by Davis (1995), who compared the effects of P. radiata and cocksfoot on soil properties in a 1-year pot experiment. A higher rate of mineralisation of soil organic matter in the presence of the pines than of the grass was indicated. Further repeated monitoring of soil total C as the trees mature is needed to authenticate the suggested difference between our tree and rank grass treatments and to establish whether, as in many mature plantations (Beets et al. 2002; Davis and Condron 2002), there is a lowering of topsoil C content under P. radiata compared with that under pasture.

In contrast to soil total C and N, microbial C and N concentrations differed markedly among treatments 15 months after tree planting, with values significantly lowest under the trees (Table 1). The greater sensitivity of microbial C than total C to land-use change has been well documented also for other pine and pasture systems (Sparling et al. 2000; Saggar et al. 2001; Ross et al. 2002). As found elsewhere (Ross et al. 1999), extractable C did not provide a reliable indication of microbial biomass or activity, neither it nor extractable organic N differing significantly among the 3 treatments.

The data in Table 1 also give an insight into processes that occur when buried bags are used to estimate N mineralisation. In these soil samples, nitrification of min-N occurred readily during the incubation, with the pH declining significantly because of protons being generated during this process. Microbial C and N concentrations, on both a dry weight and total C or N basis, were again significantly lower in the incubated cores than in the non-incubated samples. However, on a dry weight basis, there was a significant treatment x time interaction, with treatment changes being, surprisingly, negligible in the soil under rank grass but appreciable in the other treatments. Microbial C :microbial N ratios averaged 6.7 (s.e.m. 0.2) and did not change significantly with either treatment or incubation time.

This decline in microbial biomass in bare soil under the trees and during the in situ incubations is likely to have arisen from the depletion of readily metabolisable substrates (Saggar et al. 2001), and is consistent with the decline in microbial C observed in stored grassland soils (Ross 1990; Lovell and Jarvis 1998). The lowered microbial demand for min-N because of C substrate limitations, together with the low N demand by the young trees in the herbicide-treated soil, can probably account for the ready nitrification of the ammonium-N produced and leaching of nitrate-N in the early stages of tree establishment.

Conclusions

Large changes in the soil N cycle occurred with heavy grazing, herbicide application, and the absence of vegetation under the young pine trees. As hypothesised, the absence of vegetation led to enhanced net N mineralisation, nitrification, and N leaching, because of reduced plant uptake of N. For the total land area, the leaching under the young trees was, however, only 5 kg N/ha compared with 13 kg N/ha under the rank grass that was heavily grazed. Topsoil C also appeared to be lower under the young pine trees, possibly because of reduced inputs of substrate.

Acknowledgments

We are grateful to M. Krausse, M. Osborne, N. Rodda, G. Salt, J. Townsend, R. Webster, and H. Wilde for their assistance. This work was supported under Foundation for Research, Science and Technology contract CO4X0012.
Table 1. Soil pH, C, and N pools in samples taken from the three
treatments in December 2000 and after subsequent incubation of the
samples in situ for 70 days

Standard errors are given in parentheses (n = 6). Within rows,
incubation treatment values followed by the same letter are not
significantly different (P > 0.05). Where there was a difference at
P = 0.10, letters are in parentheses

Property Non-incubated soil
 Pine Rank Pasture
 grass

pH 5.50 (b) 5.75 (a) 5.71 (ab)
 (0.11) (0.01) (0.06)
Total C (g/kg) 49 a 53 a 59 a
 (1) (2) (6)
Total N (g/kg) 4.2 a 4.4 a 5.0 a
 (0.2) (0.2) (0.5)
C:N 11.8 a 12.1 a 12.0 a
 (0.2) (0.2) (0.2)
Min-N (mg/kg) 3.5 (b) 4.0 (b) 7.4 (a)
 (0.8) (0.7) (1.1)
N[O.sub.3.sup.-]-N 6 a 3 a 13 a
 (% of min-N) (2) (2) (6)
Min-N (X 1000)/ 0.89 (b) 0.96 (ab) 1.68 (a)
 total N (0.20) (0.19) (0.29)
Extractable C 217 a 184 a 181 a
 (mg/kg) (32) (5) (9)
Extractable organic 26 a 21 a 26 a
 N (mg/kg) (6) (2) (2)
Microbial C 846 b 1050 ab 1190 a
 (mg/kg) (62) (30) (100)
Microbial N 129 b 168 a 182 a
 (mg/kg) (12) (7) (13)
Microbial C/ 6.7 a 6.3 a 6.5 a
 microbial N (0.5) (0.2) (0.4)
Microbial C (X 100)/ 1.72 b 1.99 a 2.03 a
 total C (0.09) (0.04) (0.06)
Microbial N (x 100)/ 3.38 a 4.01 a 4.05 a
 total N (0.27) (0.16) (0.28)

Property Incubated soil
 Pine Rank Pasture
 grass

pH 4.96 a 5.23 a 4.92 a
 (0.12) (0.12) (0.13)
Total C (g/kg) 47 a 57 a 56 a
 (1) (2) (3)
Total N (g/kg) 4.2 a 4.8 a 4.9 a
 (0.2) (0.3) (0.6)
C:N 11.3 a 11.8 a 11.4 a
 (0.2) (0.2) (0.1)
Min-N (mg/kg) 64 a 89 a 151 a
 (11) (27) (32)
N[O.sub.3.sup.-]-N 82 a 66 a 84 a
 (% of min-N) (4) (7) (3)
Min-N (X 1000)/ 14.9 a 17.1 a 28.9 a
 total N (2.4) (5.3) (4.8)
Extractable C 197 a 217 a 238 a
 (mg/kg) (15) (10) (20)
Extractable organic 7 a 12 a 9 a
 N (mg/kg) (2) (3) (3)
Microbial C 704 b 1000 a 856 b
 (mg/kg) (45) (67) (104)
Microbial N 99 b 164 a 104 b
 (mg/kg) (4) (19) (13)
Microbial C/ 7.1 a 6.7 a 6.8 a
 microbial N (0.3) (0.5) (0.2)
Microbial C (X 100)/ 1.48 a 1.78 a 1.60 a
 total C (0.12) (0.10) (0.08)
Microbial N (x 100)/ 2.38 a 3.31 a 2.65 a
 total N (0.11) (0.48) (0.13)

Property Overall significance of treatment
 (Tr) and sampling time effects
 Treatment Time Tr x time

pH n.s. *** n.s.

Total C (g/kg) n.s. n.s. n.s.

Total N (g/kg) n.s. n.s. n.s.

C:N n.s. *** n.s.

Min-N (mg/kg) * *** n.s.

N[O.sub.3.sup.-]-N [dagger] *** n.s.
 (% of min-N)
Min-N (X 1000)/ [dagger] *** n.s.
 total N
Extractable C n.s. [dagger] [dagger]
 (mg/kg)
Extractable organic n.s. *** n.s.
 N (mg/kg)
Microbial C * *** *
 (mg/kg)
Microbial N ** *** *
 (mg/kg)
Microbial C/ n.s. n.s. n.s.
 microbial N
Microbial C (X 100)/ * *** n.s.
 total C
Microbial N (x 100)/ * *** n.s.
 total N

n.s., P> 0.10; [dagger] P< 0.10; * P < 0.05; ** P < 0.01; *** P< 0.001.


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Manuscript received 16 April 2002, accepted 18 July 2002

R.L. Parfitt (A), D.J. Ross, and L.F. Hill

Lancare Research, Private Bag 11052, Palmerston North, New Zealand.

(A) Corresponding author; email: parfittr@LandcareResearch.co.nz
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Author:Parfitt, R.L.; Ross, D.J.; Hill, L.F.
Publication:Australian Journal of Soil Research
Geographic Code:8NEWZ
Date:May 1, 2003
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