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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Factors Controlling Soil Carbon Levels in New Zealand Grasslands

Is Clay Content Important?

Landcare Research, Private Bag 11052, Palmerston North, New Zealand

percivalh{at}landcare.cri.nz

	ABSTRACT

TOP ABSTRACT INTRODUCTION Methods Results Discussion Conclusions REFERENCES

 
Soil organic matter is a major component of biogeochemical cycles and is important in maintaining soil quality. We investigated relationships between soil organic C and various soil and site properties that may influence long-term soil C accumulation across a range of soil orders in New Zealand. We used pedon and climatic data for 167 pedons under permanent grass, and carried out regression analysis between soil C (0–200 mm) contents (t ha^-1) or concentrations (g kg^-1) and climatic and soil properties, namely, precipitation, temperature, and contents or concentrations of sand, silt, clay, pyrophosphate-extractable Al (Al_py), Fe oxide, and allophane. Soil clay content or concentration explained little of the variation in soil C across all soils (R² < 0.05) and within each soil type. Likewise, mean annual precipitation and temperature explained little variation in soil C content or concentration (R² < 0.15 for precipitation, for temperature). Allophane content or concentration was unrelated to soil C in the soils of volcanic origin; Al_py, however, correlated strongly with both soil C content and soil C concentration across all soil types ( , respectively). When all factors were combined in a multiple regression analysis, the combination of Al_py and allophane contents explained the greatest amount of variation in soil C content , whereas the combination of Al_py, Fe oxide, allophane, and clay concentrations explained the greatest amount of variation in soil C concentration . Our results suggest that in New Zealand soils, chemical stabilization of organic matter is the key process controlling soil C accumulation, and that clay content relates poorly to long-term soil organic C accumulation.

Abbreviations: APPT, mean annual precipitation • APPT² the square of APPT • MAT, mean annual temperature • MAT², the square of MAT

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Methods Results Discussion Conclusions REFERENCES

 
SOIL ORGANIC C (hereafter referred to as soil C) is important as a key indicator of soil quality (Doran et al., 1994) and for sustaining agricultural production. Soil C is also the largest terrestrial C reservoir (Schlesinger, 1997), containing about twice as much C as the atmospheric CO₂ pool. Because of the important role of soil C in terrestrial ecosystems and its large reservoir size, small changes in soil C resulting from perturbations such as changes in climate, land-cover, or management may influence both long-term ecosystem sustainability and the global C budget. To predict changes in soil C, information on the key processes controlling soil C stabilization is required at a range of spatial and temporal scales.

Several soil C simulation models have been developed to describe soil C cycling processes at a range of spatial and temporal scales (Smith et al., 1997). Many of these models incorporate soil texture (especially clay content or concentration) as a key factor that controls soil C stabilization. Much of the evidence for the direct effect of clay content on soil C accumulation arises from short-term laboratory incubations (e.g., Sorenson, 1981; Saggar et al., 1994). Scaling up of these short-term processes to predict long-term soil C accumulation may, however, lead to significant soil C prediction errors (Hyvönen et al., 1998). Direct evidence for long-term effects of clay content on soil C are based on correlations between soil C and clay content in large soil C databases (e.g., Burke et al., 1989; Kern, 1994; Homann et al., 1998).

Many factors interact to control the size and turnover rate of soil C (Oades, 1988; Sollins et al., 1996). Rainfall and temperature regimes are perhaps the most important factors influencing soil C, but large-scale patterns related to climate can be modified by the quality and distribution of organic inputs and soil organomineral interactions. Several studies have found strong correlations between soil C content and clay content (Nichols, 1984; Spain, 1990; Arrouays et al., 1995; Alvarez et al., 1998), but the importance of clay could vary across soils with different clay mineralogy (e.g., basaltic and non-basaltic soils; Spain 1990). Other work has shown that climate controlled soil C storage (McDaniel and Munn, 1985; Sims and Nielsen, 1986; Burke et al., 1989; Homann et al., 1995; Alvarez et al., 1998), especially under cooler, wetter conditions (McDaniel and Munn, 1985). In forest ecosystems in Maine, soil drainage class explained much of the variation in soil C, and clay content was unrelated to soil C (Davidson and Lefebvre, 1993). None of these studies, however, examined the relationship between Fe and Al and soil C content, in spite of the potential importance of hydrous oxides (including allophane) on soil C accumulation (Sollins et al., 1996; Torn et al., 1997).

New Zealand is an ideal location to examine the effects of multiple factors on soil C. Over a relatively small area (26 million ha), there is a large range of climates. Mean annual precipitation varies from 400 to 5000 mm, and mean annual temperature is between 7 and 16°C. In addition, there is a range of soils that represents most of the major soil orders (Hewitt, 1998). Historically, most of the New Zealand landscape was forested until about 800 AD (McKinnon, 1997). After this time and up until the late 1800s, about two thirds of the forested area was removed. Most of the forest was cut and burned, then oversown with introduced grasses for grazing. Superphosphate fertilizer was added to most of these soils, and lime was applied to flatter land. Presently, about 60% of the New Zealand landscape is occupied by either improved or unimproved pasture.

The objective of this work was to examine the factors controlling soil C in New Zealand grassland ecosystems across a range of climatic conditions and soil types. We hypothesized a positive relationship between soil clay content (or concentration) and soil C content (or concentration) across soils with different mineralogy, including soils containing various amounts of allophane, Al, and Fe oxides. To this end we included chemical parameters in our data set, such as measures of allophane, Al, and Fe contents (or concentrations) in soils.

	Methods

TOP ABSTRACT INTRODUCTION Methods Results Discussion Conclusions REFERENCES

 
Database Description
Fifteen soil orders are represented in the New Zealand Soil Classification System (Hewitt, 1998). The major soil orders include the Brown, Podzol, Pallic, and Recent orders. The USDA equivalents from Soil Taxonomy (Soil Survey Staff, 1999) are shown in Table 1 .

Mean annual precipitation for the sites associated with each pedon was taken from the soils database. Mean annual air temperature was obtained for each site from a climate database for New Zealand (D.J. Giltrap, unpublished data, 1993).

Calculation of Soil Properties
Soil C content (t ha^-1) was calculated by horizon and was based on data for soil C (g kg^-1, dry weight basis), dry bulk density (B, t m^-3), volumetric stones (S, v v^-1) and the horizon thickness (t, cm) using the following equation (Tate et al., 1997):

We then summed the appropriate horizons to give us soil C content for the 0- to 200-mm depth interval. Where volumetric stone measurements were not available, values were estimated from the stone abundance in the field profile description. We also calculated the corresponding masses (t ha^-1) for Fe_d, Si_ox, Al_ox, Al_py, silt, and clay using a similar approach. Summary information for each soil order is shown in Table 2 . Many of the pedon descriptions did not contain the complete set of required data, and these were excluded from the analysis. A total of 122 pedons contained all the information required to calculate soil C content and all the property values.

Non-linear relationships were also examined, such as the power law relationship used by Alvarez et al. (1998) in regressions of soil C content vs. climatic and soil property factors. Nevertheless, these did not produce sufficient improvements in R² values to justify going beyond linear single or multiple relationships.

	Results

TOP ABSTRACT INTRODUCTION Methods Results Discussion Conclusions REFERENCES

 
Total organic soil C ranged from 34 t ha^-1 in the Semiarid Soils to 128 t ha^-1 in the Allophanic Soils (Table 2). Similar trends in soil C concentration were also observed (Table 3).

Linear Regression Analysis
Texture
Mean clay concentration ranged from 38 g kg^-1 for Podzols to 329 g kg^-1 for Gley Soils (Table 3). Pallic Soils, generally formed in aolean materials, had the highest silt concentration (655 g kg^-1). Soil C content correlated poorly with both clay and silt content across all soils (R² < 0.15) (Table 4 , Fig. 1a) . Clay content also explained <1% of the variation in soil C content within each soil order, except for the Pumice Soils (Table 4). Clay concentration and soil C concentration also correlated poorly; clay concentration accounted for only 1% of the variation in soil C across all soil types (Table 5 , Fig. 2a) .

View larger version (31K): [in this window] [in a new window]   Fig. 1 Relationships between various soil properties and soil organic C (t ha-1): (a) clay – all pedons; (b) Siox – all pedons; (c) Siox – Allophanic soil; (d) Alpy – all pedons; (e) Alpy – Allophanic soils; (f) Alpy – Brown soils; (g) Alpy – Gley soils; (h) Alpy – Pallic soils; (i) Alpy – Recent soils. R2 values and levels of significance are given in Table 4

View larger version (30K): [in this window] [in a new window]   Fig. 2 Relationships between various soil properties and soil organic C (g kg-1): (a) clay – all pedons; (b) Siox – all pedons; (c) Siox – Allophanic soils; (d) Alpy – all pedons; (e) Alpy – Allophanic soils; (f) Alpy – Brown soils; (g) Alpy – Gley soils; (h) Alpy – Pallic soils; (i) Alpy – Recent soils. R2 values and levels of significance are given in Table 5

Chemical Properties
The Oxidic Soil had the largest value of Fe_d (Table 3), a measure of crystalline Fe oxides (Parfitt and Childs, 1988), and the Allophanic Soils had the highest mean values for Si_ox, which is a measure of soil allophane content (Parfitt, 1990). Across all soils, soil C correlated positively with Si_ox (Fig. 1b and 2b), but within the Allophanic Soils, Si_ox and soil C were unrelated (Fig. 1c and 2c; Tables 4 and 5). Likewise, variation in Fe_d explained little of the variation in soil C (Tables 4 and 5).

The concentrations of Al_ox and Al_py were similar in most non-allophanic soils, indicating that the Al is associated with soil organic matter (Al–humus) (Parfitt and Childs, 1988). It should be noted that Al_py is not a measure of Al toxicity. The Al_ox arises from both allophane and Al–humus complexes. Al_py values were highest in the Allophanic Soils and lowest in the Semiarid Soils (Table 2). Al_py related strongly to soil C (Fig. 1d and 2d), and explained the largest amount of variation in soil C content and soil C concentration across all soils (Tables 4 and 5). It also explained the greatest amount of soil C variation within each soil type (Tables 4 and 5, Fig. 1e–i and 2e–i).

Multiple Regression Analysis
For all the soils combined, multiple regression analysis did not greatly improve the amount of variation in soil C content explained by Al_py alone (Table 4); adding Si_ox to the regression model increased R² to 0.57. For the Allophanic Soils alone, the multiple regression of Al_py, Fe_d, and precipitation against soil C content gave , compared with for Al_py alone (not shown in Table 4). For all other soil orders, no multiple regression model was significant.

For soil C concentration (all soils combined), the regression model including Al_py, Fe_d, Si_ox, and clay increased for Al_py alone (Table 5). For the Allophanic Soils, the multiple regression of Al_py, Fe_d, and precipitation against soil C content gave , compared with for Al_py alone (not shown in Table 5). For the other individual soil orders, the multiple regression models were not significant, except for Al_py and clay , and Al_py and Fe_d in the Gley Soils (not shown in Table 5).

	Discussion

TOP ABSTRACT INTRODUCTION Methods Results Discussion Conclusions REFERENCES

 
Overall, both soil clay content and climatic factors (precipitation and temperature) explained little of the variation in soil C for New Zealand grasslands. One possible explanation is that soil C had not yet reached a new steady state since the large-scale conversion of forest to pasture that occurred in the late 1800s and early 1900s (McKinnon, 1997). We do not believe this explains our results because (i) Tate et al. (1997) measured soil C concentration changes in 43 soils under New Zealand pasture measured 30 to 50 yr previously. They found no evidence for any changes in soil C during this time span, supporting the idea that soil C is presently near steady-state for these grassland soils. Furthermore, (ii) in soils of the Great Plains, Burke et al. (1989) found that clay and precipitation were important as predictors of soil C under both grasslands vegetation and grasslands converted to cropping, suggesting near steady-state was also achieved after decades of cropping.

Our R² values were compared with those obtained for shortgrass prairie soils using the six predictors of Burke et al. (1989). The six predictors were mean annual temperature (MAT) and its square (MAT²), mean annual precipitation (APPT) and its square (APPT²), APPT x silt, and APPT x clay. Using this equation, Burke et al. (1989) explained about 54% of the variation in soil C content in the 0- to 200-mm soil layer. Application of these six predictors to our soil C data gave an R² value of 0.34 for the combined orders, well below the R² values obtained using Al_py (Table 4). None of the six predictors in this multiple regression equation was significant when applied to individual soil orders.

A possible explanation for the poor relationship between clay and soil C (Table 4) is that the dispersion method used to measure clay did not adequately disperse soils containing allophane. Many New Zealand soils contain some allophane, and it is widely recognized that clay is not easily dispersed in Allophanic Soils (Karube and Abe, 1998). Because of this circumstance, we tested whether our clay content data were reliable. In soils where allophane was present, we confirmed that the clay had been effectively dispersed by comparing the clay content in those soils (Table 3) with the allophane content estimated from Si_ox (Parfitt, 1990). Soil clay contents were always greater than the allophane contents, confirming that dispersion had been largely achieved before measurement of clay content.

The poor correlation of clay content with total soil C is not inconsistent with the literature reviewed by Sollins et al. (1996), and for pasture soils Hassink (1994) found no significant relationship between texture and C content, except for soils with high water tables. We, on the other hand, found no such relationship for the poorly drained soils in our data set (data not shown). Oades (1988) pointed out that "while many workers agree that clay plays a role in stabilizing organic matter in soils, it is difficult to find unequivocal evidence to support such a statement. This is because clay content is usually correlated with other factors and it is not clear which factors are causative, and clay content is often correlated with greater plant growth for chemical and physical reasons and results in greater annual input of C." Oades also added that "one cannot assume that correlations between organic matter contents and high base status and clay content are causative." Generally, it is the decomposition of freshly added plant material that is influenced by clay content (Oades, 1988; Scott et al., 1996).

Pyrophosphate-extractable Al, in contrast, correlated highly with soil C (Tables 4 and 5). Al_py is thought to arise from the dissolution and dispersion of Al and Al hydroxides associated with organic matter, generally referred to as Al–humus complexes (Parfitt and Childs, 1988). Kaiser and Zech (1996) showed that pyrophosphate reagent can disperse Al hydroxide and gibbsite that is associated with organic matter. Generally, there is little gibbsite in New Zealand soils, so the pyrophosphate-extractable Al probably gives a measure of Al in a range of Al hydroxides associated with soil organic matter.

It is a matter of speculation whether C influences the Al_py values by way of soil organic matter providing binding sites for Al (Skjemstad, 1992), or whether Al stabilizes soil organic matter by altering its solubility, conformation, and surface properties (Sollins et al., 1996; Brynhildsen and Rosswall, 1997). Both possibilities are consistent with the literature.

It is known that humic acids contain large numbers of carboxyl groups that are able to complex Al (Jardine and Zelazny, 1996; Vance et al., 1996), and that Al ions strongly enhance the sorption of humic acid on layer silicate clays through the formation of Al bridges between the clay and organic matter (Varadachari et al., 1991). Alternatively, Boudot et al. (1986, 1989) showed that organo–Al associations can protect organic matter from microbial breakdown, and the mineralization rate was lower where the Al:C ratios were relatively high (Boudot, 1992). Several other studies have suggested an important role for Al in the stabilization of soil C. Skjemstad (1992) demonstrated the importance of Al:C ratios in the stabilization of soil organic matter in Podzols. In a study of decomposition of forest soil C in Costa Rica, Veldkemp (1994) found Al_py was stabilizing the pool of organic matter that was turning over in a 25-yr time frame. Likewise, in a New Zealand climosequence of soils under tussock grassland, two soils with very high extractable-Al values had very slow soil C turnover as indicated by thermonuclear bomb ¹⁴C measurements (Tate, 1992).

Allophanic Soils (Andisols) generally have high soil C contents, and in our data set the mean value for the 0- to 200-mm soil depth was 128 t ha^-1 (Table 2). It has been suggested that these high organic C contents are related to the allophane content of the soil (Boudot, 1992). For these soils, however, we found a very poor correlation between allophane content and soil C (Table 4). The high soil C contents in Andisols could have arisen from good soil physical conditions, large total P contents, and the resulting high forest–net primary productivity, all of which give substantial C inputs.

Boudot et al. (1986) and Saggar et al. (1994) found that the turnover of freshly added carbohydrate is slower in soils containing allophane because the allophane stabilizes both microbial biomass and their substrates. These results, however, were obtained from short-term incubations where C inputs were maintained. On the other hand, our data pertains to total soil C that is largely humified C, and this soil C pool may be stabilized less by allophane and more so by Al that is extractable in pyrophosphate. This is consistent with the results of Boudot (1992), who showed that Al bound in insoluble organo–Al complexes was more effective than allophane in retarding biodegradation of citric acid.

The question arises whether these results for New Zealand soils are applicable to soils globally. Most New Zealand soils formed under forest vegetation, which was replaced by grasses and legumes in the 1800s. The soils generally are acid, with pH values of A horizons commonly between 5 and 6.5. About 65% of the land area has soils with one of the subsoil horizons having a pH of <5.0 (Parfitt and McDonald, 1992). Under these conditions Al dissolves from soil minerals and is available for complexation with carboxyl groups in soil organic matter (Vance et al., 1996; Wesselink et al., 1996). Powell and Hawke (1995) showed that both acidity and Al complexing ligands are generated in New Zealand soils under high rainfall and where the native vegetation was forest. This is particularly true in soils under native Nothofagus spp., forests that dominated New Zealand's upland native forest area. Mineral weathering is very intense under this vegetation type compared with tussock grasses (Churchman, 1980), and it can lead to high levels of extractable-Al that may complex with soil humus. This mechanism of soil C stabilization was proposed to explain the exceptionally slow soil C turnover observed in some New Zealand grassland soils (Tate, 1992). Such a mechanism might be important globally in areas previously dominated by forests but now used for agriculture. It could also influence the response of these soils to changes in land use or management, particularly when these changes may alter the stability of Al–humus complexes.

	Conclusions

TOP ABSTRACT INTRODUCTION Methods Results Discussion Conclusions REFERENCES

 
For New Zealand soils under pasture, clay and silt were not good predictors of soil C (Tables 4 and 5). Neither were precipitation, air temperature, or Fe oxides. Allophane content (as indicated by Si_ox) was correlated with soil C across all soil orders, but not within the Allophanic Soils themselves. Both across and within soil orders, however, Al_py explained the greatest variation in soil C. This may be due to the historical legacy of forest vegetation that once occurred throughout New Zealand, and our findings underscore the importance of land-use history as a factor influencing soil C storage. Further work is required to quantify the effects of Al on chemical complexation reactions with soil humus and physical protection of soil organic matter within aggregates, as well as to assess the contribution of these processes to long-term soil organic matter stabilization (Sollins et al., 1996).Alvarez Lavado 1998

	ACKNOWLEDGMENTS

 
We are grateful for assistance with the climate database provided by Janice Willoughby and Hamish Heke of Landcare Research New Zealand Ltd. We acknowledge the pedologists and analysts of the former Soil Bureau, DSIR, New Zealand, for the samples and analyses done in a previous era. The work was supported under Foundation for Research, Science and Technology, New Zealand, contracts C09811 and C09817.

Received for publication July 3, 1999.

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