Marco Antonio Rondón

Scope and Strategies for Regulation of Nitrification in Agricultural Systems—Challenges and Opportunities

Nitrification, a microbial process, is a key component and integral part of the nitrogen (N) cycle. Soil N is in a constant state of flux, moving and changing chemical forms. During nitrification, a relatively immobile N-form (NH 4 +) is converted into highly mobile nitrate-N (NO 3 −). The nitrate formed is susceptible to losses via leaching and conversion to gaseous forms via denitrification. Often less than 30% of the applied N fertilizer is recovered in intensive agricultural systems, largely due to losses associated with and following nitrification. Nitrogen-use efficiency (NUE) is defined as the biomass produced per unit of assimilated N and is a conservative function in most biological systems. A better alternative is to define NUE as the dry matter produced per unit N applied and strive for improvements in agronomic yields through N recovery. Suppressing nitrification along with its associated N losses is potentially a key part in any strategy to improve N recovery and agronomic NUE. In many mature N-limited ecosystems, nitrification is reduced to a relatively minor flux. In such systems there is a high degree of internal N cycling with minimal loss of N. In contrast, in most high-production agricultural systems nitrification is a major process in N cycling with the resulting N losses and inefficiencies. This review presents the current state of knowledge on nitrification and associated N losses, and discusses strategies for controlling nitrification in agricultural systems. Limitations of the currently available nitrification inhibitors are highlighted. The concept of biological nitrification inhibition (BNI) is proposed for controlling nitrification in agricultural systems utilizing traits found in natural ecosystems. It is emphasized that suppression of nitrification in agricultural systems is a critical step required for improving agronomic NUE and maintaining environmental quality.

CARBON AND NUTRIENT ACCUMULATION IN SECONDARY FORESTS REGENERATING ON PASTURES IN CENTRAL AMAZONIA

Over the past three decades, large expanses of forest in the Amazon Basin were converted to pasture, many of which later degraded to woody fallows and were abandoned. While the majority of tropical secondary forest (SF) studies have examined post-deforestation or post-agricultural succession, we examined post-pasture forest recovery in 10 forests ranging in age from 0 to 14 years since abandonment. We measured above- ground biomass and soil nutrients to 45 cm depth and computed total site carbon (C) and nutrient stocks to gain an understanding of the dynamics of nutrient and C buildup in regenerating SF in central Amazonia. Aboveground biomass accrual was rapid, 11.0 Mg·ha 21 ·yr 21 , in the young SFs. Within 12-14 yr, they accumulated up to 128.1 Mg/ha of dry aboveground biomass, equivalent to 25-50% of primary forest biomass in the region. Wood nitrogen (N) and phosphorus (P) concentrations decreased with forest age. Aboveground P and calcium (Ca) stocks accu- mulated at a rate of 1.2 and 29.4 kg·ha 21 ·yr 21 ; extractable soil P stocks declined as forest age increased. Although soil stocks of exchangeable Ca (207.0 6 23.7 kg/ha) and extractable P (8.3 6 1.5 kg/ha) were low in the first 45 cm, both were rapidly translocated from soil to plant pools. Soil N stocks increased with forest age, probably due to N fixation, at- mospheric deposition, and/or subsoil mining. Total soil C storage to 45 cm depth ranged between 42 and 84 Mg/ha, with the first 15 cm storing 40-45% of the total. Total C accrual (7.04 Mg C·ha 21 ·yr 21 ) in both aboveground and soil pools was similar or higher than values reported in other studies. Tropical SFs regrowing on lightly to moderately used pasture rapidly sequester C and rebuild total nutrient capital following pasture abandonment. Translocation of some nutrients from deep soil (.45 cm depth) may be important to sustaining productivity and continuing biomass ac- cumulation in these forests. The soil pool represents the greatest potential for long-term C gains; however, soil nutrient deficits may limit future productivity.

Evidence for biological nitrification inhibition in Brachiaria pastures

Nitrification, a key process in the global nitrogen cycle that generates nitrate through microbial activity, may enhance losses of fertilizer nitrogen by leaching and denitrification. Certain plants can suppress soil-nitrification by releasing inhibitors from roots, a phenomenon termed biological nitrification inhibition (BNI). Here, we report the discovery of an effective nitrification inhibitor in the root-exudates of the tropical forage grass Brachiaria humidicola (Rendle) Schweick. Named “brachialactone,” this inhibitor is a recently discovered cyclic diterpene with a unique 5-8-5-membered ring system and a γ-lactone ring. It contributed 60–90% of the inhibitory activity released from the roots of this tropical grass. Unlike nitrapyrin (a synthetic nitrification inhibitor), which affects only the ammonia monooxygenase (AMO) pathway, brachialactone appears to block both AMO and hydroxylamine oxidoreductase enzymatic pathways in Nitrosomonas. Release of this inhibitor is a regulated plant function, triggered and sustained by the availability of ammonium (NH4+) in the root environment. Brachialactone release is restricted to those roots that are directly exposed to NH4+. Within 3 years of establishment, Brachiaria pastures have suppressed soil nitrifier populations (determined as amoA genes; ammonia-oxidizing bacteria and ammonia-oxidizing archaea), along with nitrification and nitrous oxide emissions. These findings provide direct evidence for the existence and active regulation of a nitrification inhibitor (or inhibitors) release from tropical pasture root systems. Exploiting the BNI function could become a powerful strategy toward the development of low-nitrifying agronomic systems, benefiting both agriculture and the environment.

Nutrient leaching in a Colombian savanna Oxisol amended with biochar.

Nutrient leaching in highly weathered tropical soils often poses a challenge for crop production. We investigated the effects of applying 20 t ha biochar (BC) to a Colombian savanna Oxisol on soil hydrology and nutrient leaching in field experiments. Measurements were made over the third and fourth years after a single BC application. Nutrient contents in the soil solution were measured under one maize and one soybean crop each year that were routinely fertilized with mineral fertilizers. Leaching by unsaturated water flux was calculated using soil solution sampled with suction cup lysimeters and water flux estimates generated by the model HYDRUS 1-D. No significant difference ( > 0.05) was observed in surface-saturated hydraulic conductivity or soil water retention curves, resulting in no relevant changes in water percolation after BC additions in the studied soils. However, due to differences in soil solution concentrations, leaching of inorganic N, Ca, Mg, and K measured up to a depth of 0.6 m increased ( < 0.05), whereas P leaching decreased, and leaching of all nutrients (except P) at a depth of 1.2 m was significantly reduced with BC application. Changes in leaching at 2.0 m depth with BC additions were about one order of magnitude lower than at other depths, except for P. Biochar applications increased soil solution concentrations and downward movement of nutrients in the root zone and decreased leaching of Ca, Mg, and Sr at 1.2 m, possibly by a combination of retention and crop nutrient uptake.

Nitrogen fixation by three tropical forage legumes in an acid-soil savanna of Colombia

Abstract The proportion of nitrogen derived from fixation was measured by 15N isotope dilution in three tropical grass-legume pastures on two Oxisols differing in texture over a 3 year period using three non-fixing controls. The use of the companion pasture grass, Brachiaria dictyoneura, was satisfactory as a non-fixing control when compared with a non-fixing Panicum maximum ecotype KK16 and native savanna grasses. Amounts of nitrogen fixed ranged from 0.3 to 40 kg N ha−1 12 weeks−1 during the wet season and were greatest with Stylosanthes capitata followed by Centrosema acutifolium and Arachis pintoi mainly as a result of greater legume biomass in the former compared with the latter two species. There were little or no differences amongst the three legumes in kg N fixed t legume DM−1. Amounts of nitrogen fixed per unit area decreased over the 3 year period mainly as a result of a loss of legume biomass. The % N derived from fixation (%Ndfa) was generally greater than 80% in all legumes on both soil types and there were little or no differences in %Ndfa with two levels of fertilization given at establishment. The %Ndfa did not change over time. In a separate experiment with A. pintoi, %Ndfa did decrease with increasing legume proportion in the sward. The results indicate that in tropical pastures sown on low fertility acid soils the amounts of nitrogen fixed by forage legumes are dependent on legume growth and persistence. Soil type, level of fertilization of age had little effect on the N2-fixation process as measured by %Ndfa. Based on these results approximate amounts of nitrogen fixed may be estimated by simple measurements of legume biomass-N × 0.8.

Soil mineral N dynamics beneath mixtures of leaves from legume and fruit trees in Central Amazonian multi-strata agroforests

Long term applications of leguminous green mulch could increase mineralizable nitrogen (N) beneath cupuacu trees produced on the infertile acidic Ultisols and Oxisols of the Amazon Basin. However, low quality standing cupuacu litter could interfere with green mulch N release and soil N mineralization. This study compared mineral N, total N, and microbial biomass N beneath cupuacu trees grown in two different agroforestry systems, north of Manaus, Brazil, following seven years of different green mulch application rates. To test for net interactions between green mulch and cupuacu litter, dried gliricidia and inga leaves were mixed with senescent cupuacu leaves, surface applied to an Oxisol soil, and incubated in a greenhouse for 162 days. Leaf decomposition, N release and soil N mineralization were periodically measured in the mixed species litter treatments and compared to single species applications. The effect of legume biomass and cupuacu litter on soil mineral N was additive implying that recommendations for green mulch applications to cupuacu trees can be based on N dynamics of individual green mulch species. Results demonstrated that residue quality, not quantity, was the dominant factor affecting the rate of N release from leaves and soil N mineralization in a controlled environment. In the field, complex N cycling and other factors, including soil fauna, roots, and microclimatic effects, had a stronger influence on available soil N than residue quality.

Soil organic carbon storage and dynamics after C3-C4and C4-C3vegetation changes in sub-Andean landscapes of Colombia

The soil C capture capacity and organic matter turnover rate vary according to photosynthetic pathways; therefore the evaluation of C at sites suffering changes from C3 to C4 vegetation and vice versa, is important to identify impacts of land use change on C cycle. This study aims to evaluate C storage under different land uses, and soil C dynamics using the 13C technique to identify the origin of soil C. In the Municipality of Alcala, Department of Valle del Cauca, Colombia, the natural abundance of δ13C was estimated, and data on land use history were gathered to calculate the organic matter turnover rate. The contribution of each type of vegetation to total percentage organic C and to storage at 0.30 m was estimated at sites suffering changes from C3 to C4 vegetation and vice versa. Average δ13C ranged between -25.79 and -20.72‰ at the three depths evaluated. Over a period of 13 yr, mature fallow lands replaced more than 70% of the C fixed by pastures over a period of 60 yr, whereas paddocks, over a period of 17 yr, only managed to replace 37.9% of the C fixed by associated coffee plantations during a period of 50-100 years. We conclude that the use of 13C avoided that C storage would have been attributed to current land uses when they are actually fixed by previous vegetation; and that C deposit from C3 vegetation is recalcitrant, while that corresponding to C4 vegetation has a relatively fast turnover rate.