Pekka Nygren

Nitrogen-fixation dynamics in a cut-and-carry silvopastoral system in the subhumid conditions of Guadeloupe, French Antilles

This paper summarizes several studies on N recycling in a tropical silvopastoral system for assessing the ability of the system to increase soil fertility and insure sustainability. We analyzed the N2 fixation pattern of the woody legume component (Gliricidia sepium), estimated the recycling rate of the fixed N in the soil, and measured N outputs in tree pruning and cut grass (Dichanthium aristatum). With this information, we estimated the N balance of the silvopastoral system at the plot scale. The studies were conducted in an 11-year-old silvopastoral plot established by planting G. sepium cuttings at 0.3 m × 2 m spacing in natural grassland. The plot was managed as a cut-and-carry system where all the tree pruning residues (every 2-4 months) and cut grass (every 40-50 days) were removed and animals were excluded. No N fertilizer was applied. Dinitrogen fixation, as estimated by the 15N natural abundance method, ranged from 60-90% of the total N in aboveground tree biomass depending on season. On average, 76% of the N exports from the plot in tree pruning (194 kg [N] ha–1 yr–1) originated from N2 fixation. Grass production averaged 13 Mg ha–1 yr–1 and N export in cut grass was 195 kg [N] ha–1 yr–1. The total N fixed by G. sepium, as estimated from the tree and grass N exports and the increase in soil N content, was about 555 kg [N] ha–1 yr–1. Carbon sequestration averaged 1.9 Mg [C] ha–1 yr–1 and soil organic N in the 0-0.2 m layer increased at a rate of 166 kg [N] ha–1 yr–1, corresponding to 30% of N2 fixation by the tree. Nitrogen released in nodule turnover (10 kg [N] ha–1 yr–1) and litter decomposition (40 kg [N] ha–1 yr–1) contributed slightly to this increase, and most of the recycled N came from the turnover or the activity of other below-ground tree biomass than nodules.

Nodulation and dinitrogen fixation of legume trees in a tropical freshwater swamp forest in French Guiana

Nodulated legume trees comprised 43% of the stand basal area in the low, most frequently flooded microsites, and 23% in higher, drier microsites in a tropical freshwater swamp forest in French Guiana. Dinitrogen fixation in Pterocarpus officinalis , Hydrochorea corymbosa and Inga pilosula was confirmed by acetylene reduction assay (ARA), presence of leghaemoglobin in nodules and the 15 N natural abundance method. The results for Zygia cataractae were inconclusive but suggested N 2 fixation in drier microsites. Nodulated Inga disticha had a 15 N-to- 14 N ratio similar to non-N 2 -fixing trees, but ARA indicated nitrogenase activity and leghaemoglobin was present in nodules. All bacterial strains were identified as Bradyrhizobium spp. according to the partial 16S rDNA sequences, and they were infective in vitro in the model species Macroptilium atropurpureum . About 35-50% of N in the leaves of P. officinalis , H. corymbosa and I. pilosula was fixed from the atmosphere. Dinitrogen fixation was estimated to contribute at least 8-13% and 17-28% to whole-canopy N in high and low microsites, respectively. Symbiotic N 2 fixation appears to provide both a competitive advantage to legume trees under N-limited, flooded conditions and an important N input to neotropical freshwater swamp forests.

Fine root and nodule dynamics of Erythrina poeppigiana in an alley cropping system in Costa Rica

Fine root and nodule production and turnover in pruned 2- and 8-yr-old Erythrina poeppigiana (Walp.) O.F. Cook trees were estimated under humid tropical conditions by applying the compartment flow model (CFM) to fine root and nodule biomass and necromass measured in sequentially taken core samples. Shoot pruning intensities compared were complete pruning (i.e., complete removal of shoots) and partial pruning (i.e., retention of one branch on the pruned stump). The CFM provided reasonable estimates of nodule dynamics but did not apply to fine root data. Over a five-month observation period, nodule production in completely and partially pruned 2-yr-old trees was 58.2 and 115 g tree−1, respectively, and the corresponding values in 8-yr-old trees were 26.8 and 26.4 g tree−1. Senescent nodules and fine roots pass to soil organic matter via decomposition. Partially and completely pruned 2-yr-old trees added 95.4 and 50.4 g tree−1 decomposed nodules to soil, respectively. The respective value for 8-yr-old trees were 26.7 and 36.5g tree−1. Nodule and fine root turnover was compensated for by new production at 10–14 weeks after pruning. The retention of a branch on the pruned E. poeppigiana tree stump allows better fine root and nodule survival, and enhances tree biomass production.

Role of root inputs from a dinitrogen-fixing tree in soil carbon and nitrogen sequestration in a tropical agroforestry system

Agroforestry is often mentioned as a suitable technology for land rehabilitation in the tropics and for mitigation of climate change because this land-use favours nutrient recycling and C sequestration. The aim of this work was to estimate soil C sequestration in a 12-year-old tropical silvopastoral system composed of a legume tree (Gliricidia sepium) and a C4 fodder grass (Dichanthium aristatum), and to link it with tree root biomass and N status in the soil. The site was under cut-and-carry management, i.e. tree pruning residues and cut grass were removed from the field and fed to stabled animals elsewhere. Thus, main sources for tree C and N inputs were root activity and turnover. Organic C derived from the trees and tree root biomass were determined based on natural 13C abundance. For the 0–0.2u2009m soil layer, the biomass of tree roots ≤2u2009mm diameter was 2.4u2009Mg/ha when the trees were pruned every 6 months (SS6), and 0.6u2009Mg/ha when pruned every 2 months (SS2). Both C (R2 = 0.39, P < 0.05) and N (R2 = 0.82, P < 0.05) sequestration were correlated with tree root biomass. The trees and grass contributed 18 and 8u2009Mg C/ha to soil, respectively, over the 12-year experiment in SS6. The net increase of 2.5u2009Mg N/ha in soil, originating from the trees, contributed to the net soil C sequestration. In SS2, trees contributed 16u2009Mgu2009C/ha to soil over 12 years, but grass-derived C was reduced by 2u2009Mgu2009C/ha because of the small amount of grass litter. The increase of 1.7u2009Mgu2009N/ha in soil, derived from the trees, was not large enough to avoid C loss in this plot. Differences in soil C and N sequestration between plots were due to differences in system management, which affected the amount and the C/N ratio of inputs and outputs.

Neotropical Legume Tree Inga edulis Forms N2-fixing Symbiosis with Fast-growing Bradyrhizobium Strains

Inga edulis Mart. is a tropical legume tree used for shade in coffee and cacao plantations and as a hedgerow in alley-cropping practices. Little information can be found concerning N2 fixation in this species. This study was conducted to characterize the rhizobia of I. edulis and determine if it is capable of fixing substantial amounts of N2. Four strains of fast-growing, Gram-negative rhizobia-type bacteria were isolated from I. edulis nodules. The strains were identified by sequencing of partial 16S–23S rDNA internal spacer region. Nitrogenase activity was determined using acetylene reduction assay (ARA). Dinitrogen fixation was measured under controlled conditions by the 15N isotope dilution technique using two non-N2-fixing reference species, Vochysia guatemalensis Donn. Sm, and Gmelina arborea Roxb. ex. Sm. Seedlings were grown in three growth media: native soil and naturally N-depleted sand amended to a low and high N level. The four strains of symbiotic bacteria were closely related to Bradyrhizobium japonicum and to Bradyrhizobium liaoningense. Nodules demonstrated nitrogenase activity as measured by ARA. Vochysia guatemalensis was a better non-N2-fixing reference than G. arborea. When V. guatemalensis was used as the non-N2-fixing reference, the estimate of the percentage of N fixed from atmosphere out of total N in I. edulis seedlings was ca. 40 in the two sand media treatments and 10 in the native soil.

Root exudates of a legume tree as a nitrogen source for a tropical fodder grass

Exudation of nitrogenous compounds from the roots of dinitrogen-fixing plants is a potential source of nitrogen for adjacent plants in intercropping systems. We studied (1) the extent of N exudation from the roots of a tropical legume tree Gliricidia sepium (Jacq.) Kunth ex Walp., and (2) the ability of a C4 fodder grass Dichantium aristatum (Poir) C.E. Hubbard and its mycorrhizal symbionts to absorb N from tree exudates in a glasshouse experiment. Root exudates of 15N-labelled trees were collected in hydroponic culture and applied with irrigation water on grass grown in separate pots. During the 10-week experiment, the trees exuded 34.1xa0±xa05.0xa0mg of N, which represented 1.7xa0±xa00.2% of their total N by the end of the experiment. The total amount exuded would have been enough to supply 16% of grass N content by the end of the experiment. The grass, however, absorbed only 3.8–7.5% of 15N in exudates and gained 0.8–1.1% of its N from exudates. The low absorption of exudate N by grass was explained by probable soil microbial immobilisation and by the dilution of exuded N in the substantially larger pool of soil mineral N. A close contact between the root systems of N donor and recipient plants directly or via their mycorrhizal symbionts seems to be a precondition of the apparently direct N transfer earlier observed in field studies of the same soil-plant system.