Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils

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1 Natural Resource Ecology and Management Conference Papers, Posters and Presentations Natural Resource Ecology and Management Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils James W. Raich Iowa State University, Ann E. Russell Iowa State University, Follow this and additional works at: Part of the Biogeochemistry Commons, Forest Biology Commons, Natural Resources and Conservation Commons, and the Natural Resources Management and Policy Commons Recommended Citation Raich, James and Russell, Ann. Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils. Available from Nature Precedings (2010) This Presentation is brought to you for free and open access by the Natural Resource Ecology and Management at Iowa State University Digital Repository. It has been accepted for inclusion in Natural Resource Ecology and Management Conference Papers, Posters and Presentations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact

2 Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils Abstract Secondary forests and young forest plantations frequently have high rates of tree growth and NPP higher even than mature forests in similar situations. The nutrients required to sustain this rapid growth are derived from the soil and by external inputs such as rainfall or, in the case of nitrogen, by biological N fixation. In our study of the effects of tree species within replicated experimental plantations in a moist, lowland tropical environment in Costa Rica, high rates of biomass accumulation and productivity were coupled with high rates of N accumulation and cycling. We applied extensive sampling through time within a mass-balance approach to address the question Where does all the nitrogen come from? Rates of N uptake by the vegetation in these plantations were extraordinarily high, even by tropical forest standards, reaching 412 kg N ha -1 yr -1. Rapid decay coupled to tight nitrogen cycling may have provided large amounts of available N for plant uptake, but do not explain the large quantities of N that accrued in the vegetation, up to 1075 kg N/ha over 16 years. Surface soil organic matter stocks in the plantations increased by as much as 320 kgc/ha in surface soils, but soil nitrogen varied differently. Soil N stocks to 1 m depth were depleted by an average of 2119 kg/ha relative to the mature forest. Thus, mineralization of soil organic nitrogen could have supplied the N that accrued in biomass over the 16-yr period, but this apparently occurred without concomitant net loss of soil C. The C:N ratios of soil organic matter (SOM)in the plantations indicated either replacement of SOM with more C-rich detritus or selective removal of N from existing SOM. Regardless, high productivity in these plantations apparently was supported in part by mining of soil nitrogen. Species varied, however. Depletion of soil N stocks was only 219 kg/ha under Vochysia guatemalensis, in which 1075 kgn/ha had accrued. Asymbiotic N fixation is the next most plausible mechanism for supplying plant-available N, and may be enhanced in stands with high availability of recently produced photosynthates. Keywords fertility, NPP, productivity, soil organic matter, succession Disciplines Biogeochemistry Forest Biology Natural Resources and Conservation Natural Resources Management and Policy Comments This presentation is from 2010 Annual Meeting of the Ecological Society of America (2010): doi: / npre Posted with permission. This presentation is available at Iowa State University Digital Repository:

3 Surprisingly rapid nitrogen cycling in tropical forest plantations on volcanically derived soils Jim Raich & Ann Russell Iowa State University

4 Acknowledgements NSF DEB # and # Dick Fisher and Eugenio Gonzalez, Texas A&M University Oscar Valverde, Kent State University Bill Parton, Colorado State Univ. Ricardo Bedoya, Gary Hartshorn, Liz Losos, ECOS and La Selva personnel, Organization for Tropical Studies

5 Study Site La Selva, Costa Rica 4000 mm annual rainfall, with no month averaging <150 mm. ~26 C mean annual temperature. Decent soils (Tropohumults)

6 Experiment established in

7 Sixteen years later

8 Experimental Design Randomized Complete Block (N=4) Monospecific plantations

9 High rates of C cycling Russell et al. (2010) Ecol. Appl. 20: 1087.

10 N Uptake by Vegetation N Uptake = N Flux in Fine Litterfall + N Flux in Branch fall + ΔN in Net Tree Growth + N Flux in Fine Root Turnover

11 N uptake by Vegetation Hial Pema Pipa Viko Vogu N Uptake (kg ha -1 yr -1 ) Mean N Uptake: 340 kgn ha -1 yr -1

12 N uptake by Vegetation Hial Pema Pipa Viko Vogu Puerto Rico Chestnut et al. (1999) N Uptake (kg ha -1 yr -1 ) Mean N Uptake: 340 kgn ha -1 yr -1

13 Also Large Amounts of N in Biomass Hial Pema Pipa Viko Vogu N in Biomass (kg/ha) Net annual accretion: 50 kgn ha -1 yr -1

14 Where does all this N come from? N uptake reaches: 400 kg N ha -1 yr -1 N sequestration reaches: 60 kg N ha -1 yr -1

15 N Flux Information from La Selva Atm. Deposition: 10 kgn ha -1 yr -1 Throughfall (in forest): 17 Leaching (from forest): 7 N-gas Losses:? N needed (average): kgn ha -1 yr -1 data from multiple publications

16 Soil Nitrogen Mean SON: 14,000 kg/ha Mean Biomass N: 800 kg/ha Soil N, cm (kg/ha) P = a a b b b Hial Pema Pipa Viko Vogu Forest Pasture Species/Vegetation

17 Soil Nitrogen Mining Mean SON: 14,000 kg/ha Mean Biomass N: 800 kg/ha Soil N, cm (kg/ha) P = a a b b b Hial Pema Pipa Viko Vogu Forest Pasture Species/Vegetation

18 Soil Carbon is Sequestered ΔSOC > 0: P = Pipa Pema Viko Hial Vogu Russell et al. (2007) SSSAJ Change in SOC, Mg C ha -1 yr -1

19 Soil C:N P = Pema Pipa Viko Vogu Pasture Forest a ab ab ab bc c c Species/Vegetation Hial Soil C:N (0-30 cm)

20 Summary Soil mining of N seems to be important. In contrast, the soil is sequestering C. No apparent soil mining of N by the fastest N-Cycler: Vochysia Can asymbiotic N fixation dependably provide 60 kgn ha -1 yr -1 to Vochysia? Why is there less N in the only N-fixer plots, containing Pentaclethra?

21 Thank You!