EARLY-GROWTH PARAMETERS ASSOCIATED WITH
TOLERANCE OF LOW-PHOSPHORUS FERTILITY IN ACID SOIL
OF FIVE NITROGEN-FIXING TREE SPECIES
A THESIS SUBMITTED TO THE GRADUATE DIVISION
OF THE UNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
AGRONOMY AND SOIL SCIENCES
Véronique Theresa Lambert
Harold H. Keyser, Chairperson
James H. Fownes
Paul W. Singleton
Russell S. Yost
We certify that we have read this thesis and that, in our opinion, it is satisfactory in scope and quality as a thesis for the degree of Master of Science in Agronomy.
Thank you firstly to the U.S. government and to the University of Hawaii for making this thesis possible by supporting my studies and my research at NifTAL Center.
I also thank all NifTAL staff for their tremendous support, often beyond the call of duty. I am particularly indebted to Geoff Haines, Kevin Keane, Kathy MacGlashan, and Tricia Scott. Fellow students Surya Tewari and Bruce Linquist provided invaluable advice, labor, and companionship. The Nitrogen Fixing Tree Association helped get me started with seed material and technical advice.
Finally, I extend my gratitude to my advisor, Harold Keyser, who provided good-natured assistance at all stages of my thesis work.
Phosphorus is a primary constraint to agroforestry systems on acid soils of the humid tropics. Strategies of low-P tolerance were evaluated for nitrogen-fixing tree species with potential for use in such systems. Trees were grown at different P levels in an ultisol with low P fertility. Acacia auriculiformis (A.a.) and Acacia mangium were tolerant of low P. Fast growth in field-planted A.a. at low P was associated with low internal P and N concentrations and with greater BNF efficiency per unit of nodule and per unit of plant P. Growth of Gliricidia sepium, Leucaena diversifolia, and Sesbania grandiflora was greatly restricted at low P. These species had higher leaf P and N concentrations and greater biomass fractions in stems and roots. Roots of these species had less surface area per unit dry weight, and were present in larger fractions in the top soil layer.
TABLE OF CONTENTS
Acknowledgements ............................................ 3
Abstract ................................................. 4
List of Tables ........................................... 6
List of Figures .......................................... 8
Chapter 1. Thesis Introduction .......................... 9
Chapter 2. Above and below-ground growth parameters
associated with varying degrees of low-P tolerance among
six nitrogen-fixing tree species grown in acid soil ...... 13
Chapter 3. Early growth response to phosphorus and
associated differences in root parameters of four
field-planted nitrogen-fixing tree species ............... 43
Chapter 4. Utilization efficiency of P, N, leaves,
roots, and nodules in four nitrogen-fixing tree species
in response to P in an acid soil ......................... 68
Chapter 5. Thesis Conclusion ............................ 91
Appendix A. Layout of field experiment .................. 95
Appendix B. Seedling dry weight at the time of
Transplanting ............................................ 96
Literature Cited ......................................... 97
LIST OF TABLES
2.1 Response of shoot, root, and nodule dry weight
and of nodule number to P in pot-grown trees ............. 36
2.2 Biomass partitioning to shoots, roots, and
nodules in pot-grown trees in response to P .............. 37
2.3 Specific absorption efficiency and rate of root
infection by vesicular-arbuscular mycorrhizae of
pot-grown trees in response to P ......................... 38
2.4 Whole-plant P and N accumulation in response to
P in pot-grown trees ..................................... 39
2.5 Internal P concentration of shoots, roots, and
nodules, and P utilization efficiency of pot-
grown trees in response to P ............................. 40
2.6 N2-fixed, % of plant N derived from atmospheric N,
specific nodule activity, and P efficiency of N2-
fixation of pot-grown trees in response to P ............. 41
2.7 Internal N concentration of shoots, roots, and
nodules, and N utilization efficiency in pot-
grown trees in response to P ............................. 42
3.1 Dry weight partitioning in 4- and 8-month old
field-grown trees in response to P ....................... 61
3.2 Plant component dry weights of 4- and 8-month old
field-grown trees in response to P ....................... 62
3.3 Soil P and pH before and after fertilization
of the field experiment .................................. 63
3.4 Component dry weight ratios in 4- and 8-month old
field-grown trees in response to P ....................... 64
3.5 Root radius and root length density of 4- and
8-month old field-grown trees in response to P ........... 65
3.6 Root surface area parameters in 4- and 8-month
old field-grown trees in response to P ................... 66
3.7 Root infection by vesicular-arbuscular
mycorrhizae in 4- and 8-month old field-grown
trees in response to P ................................... 67
4.1 Whole-plant P and N use efficiency in 4- and
8-month old field-grown trees in response to P ........... 82
LIST OF TABLES (continued)
4.2 Phosphorus concentration of component tissue of
4- and 8-month old field-grown trees in response
to P ..................................................... 83
4.3 Whole-plant P and N accumulation in 4- and 8-month
old field-grown trees in response to P ................... 84
4.4 Nitrogen concentration of component tissue of
4- and 8-month old field-grown trees in response
to P ..................................................... 85
4.5 N2-fixed, biological nitrogen fixation P efficiency,
and specific nodule activity in 4- and 8-month old
field-grown trees in response to P ....................... 86
4.6 Specific leaf area and net assimilation rate of
4- and 8-month old field-grown trees in response
to P ..................................................... 87
4.7 Leaf area and leaf area ratio of 4- and 8-month old
field-grown trees in response to P ....................... 88
4.8 Relative growth rate of field- grown trees at 0-4
and 4-8 months in response to P .......................... 89
4.9 Phosphorus uptake efficiency of 4- and 8-month
old field-grown trees in response to P ................... 90
LIST OF FIGURES
2.1 Whole-plant dry weight of pot-grown trees in
response to P ............................................ 34
2.2 Shoot and root growth of Sesbania grandiflora
in response to P, with and without inoculation ........... 35
3.1 Whole-plant dry weight of inoculated and
uninoculated field-grown trees, 4 and 8 months
old, in response to P .................................... 60
CHAPTER 1. Thesis Introduction
Importance of Phosphorus in Agroforestry Systems in the Humid Tropics
Agroforestry, the managed combination of tree production with that of crops or livestock, can be a viable land-use system on marginal soils in the humid tropics. Agroforestry systems fulfill various needs, including those for food, fuel-wood, or livestock feed, in areas with erodible soils and low soil fertility. Phosphorus has been identified as the nutrient of most concern to the success of agroforestry systems in tropical regions (Palm et al., 1991; Shepherd, 1991). This thesis addresses the problem of P constraints to agroforestry systems in the humid tropics by investigating strategies of low-P tolerance in nitrogen-fixing tree (NFT) species adapted to that environment.
One reason for the concern with P is the prevalence of soils with high levels of P-fixation in the tropics. Soils with high P-fixing capacities are particularly widespread in the humid tropics, accounting for 38% of the land in this region (Sanchez and Logan, 1992).
Focus on P limitation in agroforestry also results from the realization that P is necessarily exported out of agricultural systems with harvests, especially of P-rich components such as grain. Phosphorus inputs are required to sustain any system from which there are P losses. In regions where economic and infrastructural constraints forbid copious use of chemical fertilizers, employing species that are inherently well-adapted to low P fertility reduces the need for external inputs.
A third reason for concern with P in agroforestry systems is the importance of this nutrient for biological nitrogen fixation (BNF) (Cassman et al., 1980 and 1981; Gates, 1974; Israel, 1987). Nitrogen, as the most limiting nutrient in agriculture (Singer and Munns, 1987), is often a major constraint to tree and crop growth. The use of NFTs in agroforestry systems can alleviate the problem of N deficiency in soil for both trees and companion crops or livestock (Dommergues, 1987; Siaw et al., 1991; Szott et al., 1991). To realize the benefits of BNF to the system, P supply should be sufficient to maintain the BNF symbiosis.
In this thesis, low-P tolerance of NFTs is investigated in acid soil since high P-fixation is commonly associated with acid soils (Sanchez and Uehara, 1980). Soil acidity, like P infertility, is unlikely to be amended in many agroforestry systems in the humid tropics due to economic and infrastructural constraints. Therefore, acid-tolerance is often implicit in the low-P tolerance of agroforestry species. The soil used in this research, an ultisol, exhibited very low levels of plant-available P, as well as low pH, but had low Al saturation. Therefore, this research is most relevant to the smaller, yet substantial, proportion (24%) of acid soils in the humid tropics that is not constrained by Al toxicity (Sanchez and Logan, 1992). Because soil acidity in this research was unamended, species with some degree of reputed acid tolerance were selected to be tested for their low-P tolerance.
Environmental Adaptation and Uses of NFT Species Selected for Experimentation
Fast-growing, NFT species were selected first for their current or potential importance to agroforestry on marginal soils in the tropics. Other selection criteria were adaptation to lowland, humid tropics, tolerance of soil acidity, identification of effective rhizobia, and availability of seed. The six species selected are described as follows.
Acacia angustissima is found in North and Central America. A short, shrubby tree which resprouts after cutting, it has good potential for use in hedgerows, as nurse trees, and for rehabilitating degraded land (Benge, 1990).
Acacia auriculiformis and Acacia mangium are exceptionally hardy species, particularly A. auriculiformis which withstands many environmental extremes. Both species tolerate soil infertility and acidity (to pH 3 and 4 respectively). They occur naturally in humid tropical areas of Australia, Papua New Guinea, and Indonesia with annual rainfall of 1000-3000 mm and altitudes below 100 m (Turnbull, 1987a, 1987b). These species are suitable for fuelwood, wood, shade, and rehabilitation of degraded sites.
Gliricidia sepium is a widely used species that originated in Mexico and Central America. It is used to provide many products and services including shade, support, living fences, fuelwood, animal feed, and green manure. This species has broad adaptability within the humid tropics and some provenances can grow well on acid and infertile soils (Chadhokar, 1982).
Leucaena diversifolia, a native of Mexico and Central America, prefers fertile soils and cooler and wetter sites at higher elevations (700 to 2500 m). However, it does colonize lower-elevation (0-500 m) sites with higher temperatures, lower rainfall (650 mm), and low fertility, and can tolerate moderate acidity. The primary uses of this species are fuelwood, posts, pulpwood, shade, and reforestation (Bray and Sorennson, 1992).
Sesbania grandiflora, native to Southeast Asia, is adapted to the lowland (0-500 m) humid (1000-2000 mm rainfall) tropics and does not tolerate cool temperatures. It is used for fodder, green manure, pulp, shade, and human food. Some Sesbanias grow well on acid soils (NFTA, 1990).
This research was undertaken to address a need, articulated by Shepherd (1991) in a review paper, for information on the performance of NFT species on low-P sites. Species adapted to low-P conditions are required for low-input agroforestry systems, and information on their growth characteristics with low P fertility is necessary for effective species selection and management. The success of agroforestry systems depends on correctly matching NFT species with the needs of the system. For example, as Shepherd (1991) points out, a species adapted to low-P by virtue of slow growth and/or low leaf P concentration would not be effective in supplying P to companion crops.
The objectives of this thesis were, first, to determine the relative low-P tolerance of acid-tolerant NFT species; and, then to identify growth parameters associated with tolerance of and sensitivity to low P availability. Knowledge generated by this research is intended to facilitate effective selection and management of NFT species for successful establishment in P-limited systems in the humid tropics. The thesis focuses on early-growth performance since good tree establishment is critical to successful agroforestry. Trees require a longer time for establishment than most crops and often must compete with aggressive weeds.
Performance of the selected NFT species in low-P soil was assessed in the light of three strategies for plant survival of low fertility, outlined by Mulligan and Patrick (1985): 1) slow growth, 2) efficient nutrient acquisition, and 3) efficient nutrient utilization. Performance of the species was initially assessed in a pot experiment, reported in Chapter Two. Species that displayed different degrees of P responsiveness in the pot experiment were selected for further study in the field. In Chapters Three and Four, indicators of the strategies employed to tolerate low P fertility are assessed for the different species. Chapter Three assesses indicators of the species’ growth rates and efficiencies of nutrient acquisition, and investigates the association of these parameters with low-P tolerance. Chapter Four looks at the association between low-P tolerance and efficiency of P and tissue utilization.
CHAPTER 2. Above and below-ground growth parameters associated with varying degrees of low-P tolerance among six nitrogen-fixing tree species grown in an acid soil.
The objective of this study was to generate information about elements of low-P survival strategies of nitrogen-fixing tree (NFT) species with potential for use in acid soil systems. In a greenhouse pot experiment, six NFT species, Acacia angustissima (A. ang.), Acacia auriculiformis (A.a.), Acacia mangium (A.m.), Gliricidia sepium (G.s.), Leucaena diversifolia (L.d.), and Sesbania grandiflora (S.g.), were grown at 5 levels of applied P (0, 25, 75, 200, and 400 g P kg-1 soil) in an ultisol with pH 4.5. Acacia angustissima grew poorly at all P levels. Acacia auriculiformis and A.m. maintained moderate growth across P levels and were termed non-responsive to P. Leucaena diversifolia and S.g. increased biomass production at high P. They were termed most P-responsive, with biomass at 400 P being 2.3 times that of the 0 P control. Gliricidia sepium was the least P-responsive (P<0.17), with 1.3 times the biomass at 400 P as at 0 P. Acacia auriculiformis’ and A.m.’s lack of P-response was associated with slower growth, greater P uptake efficiency of roots (specific absorption efficiency (SAE), g P in plant g-1 roots), higher internal P utilization efficiency (PUE, g dry weight g-1 P in plant), greater efficiency of biological nitrogen fixation (BNF) per unit of P assimilated (BNF P efficiency (BNFPE), g N2 fixed g-1 P in plant), and higher specific nodule activity (SnA, g N2 fixed g-1 nodule). Increased P uptake by A.a. and A.m. at higher P levels resulted in elevated P concentrations internally. The higher rate of vesicular arbuscular mycorrhizae (VAM) root infection in A.a. suggests that VAM symbioses may have imparted greater low-P tolerance to the Acacia species. Biomass production was highest and shoot and root tissue P concentrations were lowest in G.s. than in any other species at all P levels. Gliricidia sepium had the highest PUE, BNFPE, and SnA. However, the degree of growth increase with added P was less in G.s. than in the other responsive species. Its P response may have been limited by the low SAE of its roots. The greater restriction of L.d.’s and S.g.’s growth by P infertility was associated with a relatively high internal P demand for growth and BNF.
The prevalence of P-deficient acid soils in the tropics (Sanchez and Logan, 1992) necessitates the utilization of nitrogen-fixing tree (NFT) species tolerant of such conditions in low-input agroforestry systems. Furthermore, the ubiquity of N limitations to agriculture (Singer and Munns, 1987) also calls for the tolerance of the biological nitrogen fixation (BNF) symbiosis to P infertility. Due to the broad diversity of agroforestry systems, information on the strategies with which NFT species cope with P deficiency is needed to improve species selection and management for these systems. Previous research has identified three primary elements of plant strategies for tolerating low fertility by maintaining low nutrient demand: 1) lower growth rates (Aerts, 1990; Blair and Wilson, 1990; Mulligan and Sands, 1988; Mulligan and Patrick, 1985), 2) efficient nutrient acquisition (Chapin, 1980; Paynter, 1993), and 3) efficient internal economies via increased efficiency in nutrient redistribution and in metabolic utilization (Crawford et al., 1991; Haynes et al., 1991; Israel and Rufty, 1988; Mulligan and Sands, 1988; Sanginga, 1994).
This paper reports on a preliminary investigation of the strategies of six NFT species for coping with P infertility in acid soils. There are two objectives. The first is to assess the low-P tolerance of six NFT species, Acacia angustissima, Acacia auriculiformis, Acacia mangium, Gliricidia sepium, Leucaena diversifolia, and Sesbania grandiflora. The second objective is to identify differences in growth parameters among the species that may account for differential tolerance to low P availability.
A significant component of low-P tolerance in plants is low demand for external P (Barber, 1984; Chapin, 1980) which can result from slow growth. For this study, it was hypothesized that species with greater low-P tolerance would have inherently slower growth rates at all P levels. Others (Aerts, 1990; Chapin, 1980; Mulligan and Sands, 1988) have shown that genotypes adapted to low fertility had slow growth and did not respond to improved fertility. Plants adapted to high fertility likewise will often display reduced growth under a nutrient stress, but possess the potential to increase growth should fertility improve (Aerts, 1990; Asher and Loneragan, 1967; Mulligan and Sands, 1988; Sanginga, 1992).
In addition to slow growth, species tolerant of low P fertility may employ other factors to maintain low soil P demand. One key element of a strategy of low P demand is a high PUE. Crawford et al. (1991) observed lower P concentrations in pine trees when unfertilized. They also found comparatively lower P concentrations in pine families that were more tolerant of soil infertility. However, some species adapted to high fertility conditions that have fast growth rates may actually produce biomass at a lower nutrient cost (Chapin, 1980). Such was the case for fast-growing deciduous grass from fertile sites studied by Aerts (1990). Compared to slow-growing evergreen shrubs adapted to poor fertility, the deciduous species produced more biomass per unit of P assimilated. Mulligan and Sands (1988) also found that under nutrient-limiting conditions, Eucalyptus species adapted to low-fertility sites had higher tissue P concentrations than species from more-fertile sites.
Demand for fertilizer-P may also be reduced through effective symbiosis with mycorrhizae (Mosse, 1981). For a given fertilizer application, higher rates of VAM infection could result in greater P uptake. Another factor in the strategy to maintain low demand for fertilizer-P can be a higher root efficiency for P uptake at low levels of soil P (Paynter, 1993). However, as demonstrated by Blair and Wilson (1990) in a comparison of white clover accessions, adaptation to low P fertility is not necessarily related to greater efficiency in P uptake.
For the current study, it was hypothesized that those species displaying greater tolerance of low-P fertility would have higher PUE, higher VAM infection rates, and greater SAE.
The P efficiency of the BNF symbiosis is crucial too in systems that are limited by N as well as by P. Phosphorus serves a critical role in BNF (Cassman et al., 1981), and, in agroforestry systems on low-fertility sites, NFTs are commonly expected to be at least self-sufficient in N. Some authors have concluded that the restriction of nodulation and BNF at low P occurs because host plant growth is first restricted (Robson, 1983, Reddell et al., 1988). But others have observed, rather, that a P deficiency can restrict nodulation and BNF to a greater extent than plant growth (Cassman et al., 1980, 1981; Israel, 1987; Pongsakul and Jensen, 1991). A P deficiency can also inhibit nodule function. Gates (1974) found that nodules fixed less N2 when P supply was low. In light of the importance of BNF in the N nutrition of NFTs growing in infertile soil, it was hypothesized that tolerance of P infertility would require a BNF symbiosis that is also low-P tolerant.
The effect of P on the BNF symbiosis itself was assessed through indirect analyses by: 1) calculating P efficiency of BNF (BNFPE), i.e., the amount of N fixed per unit of absorbed P; and 2) calculating specific nodule activity (SnA), i.e., the amount of N fixed per unit of nodule dry weight.
The degree to which plant growth is affected by reduced fertility varies by genotype and is associated with such factors as biomass and nutrient partitioning. Phosphorus deficiency often results in relatively less biomass and P allocation to shoots and more to roots (Fredeen et al., 1989; Israel and Rufty, 1988; Mulligan and Sands, 1988; Pongsakul and Jensen, 1991). In the case of N2-fixing plants, partitioning to nodules also plays a significant role in plant response to P. Cassman (1980) observed that P-deficient soybeans allocated biomass preferentially to roots, to the detriment of nodule development. Restricted nodule development can inhibit growth of plants that are dependent on BNF as a N source. In the current study, the expectation was that species adapted to low-P conditions would exhibit smaller increases in biomass partitioning to roots at the expense of shoots and nodules.
In this experiment, P-responsiveness was first determined from total biomass response to P availability by six NFT species. Then above and below-ground growth parameters associated with the species’ P responsiveness were assessed as elements of possible strategies for coping with low P. Parameters assessed were biomass partitioning, P uptake, P partitioning, N2 fixation, and efficiency of the following: P and N use, P uptake, nodule function, and BNF.