Nitrogen fixation during decomposition of sugarcane (Saccharum officinarum) is an important contribution to nutrient supply in traditional dryland agricultural systems of Hawai'i

Abstract

Nitrogen (N) is unique among essential elements required for life, in that it must be fixed from the vast atmospheric reservoir before most organisms can use it. Prior to industrial nitrogen fixation, many agricultural systems were limited in their productivity by N. What sustained N in the ancient Hawaiian dryland (rain-fed) agricultural systems that lacked legumes or other known significant N inputs? N-fixation during sugarcane (Saccharum officinarum) growth and litter decomposition was examined in settings representing pre-European Hawaiian agriculture. We did not detect associative N-fixation during the growth of five Hawaiian sugarcane cultivars. In contrast, N-fixation during the decomposition of leaf and stalk material was important. We found that the depth of the mulch layer significantly affected N-fixation levels during decomposition; values of N-fixation in different depths of senesced leaf litter ranged from 0.69 to 1.36 gN/kg of litter integrated over the lifetime of decomposition. Compared to senesced leaf litter, N-fixation during decomposition of nonsenesced leaf litter was 77% and stalk material 140% per unit mass. Peak rates of N fixation occurred between 200 and 400 days of decomposition, and ranged from 1.37 to 3.27 gN/kg/yr. Our empirical results were extrapolated to represent the traditional Hawaiian cropping system; we calculated N-fixation inputs of 4.8-39.0 kgN/ha/yr, with fixed N adding 17-40% of the amount of N added through litter. Findings indicate that significant N may be introduced into natural cropping systems through mulching practices and that small changes in practices greatly alter the total inputs. The use of mulch was likely an important source of N in pre-industrial settings and may be used in contemporary systems to reduce nitrogen fertilizer requirements.

Rain, Sun, Soil, and Sweat: A Consideration of Population Limits on Rapa Nui (Easter Island) before European Contact

“…The average population increases by a factor of 6.6 entirely as a result of a 4.2-times increase in the N-fixation rate. At this point we do not have evidence that points clearly to one end of the N-fixation spectrum or the other, although we do know that varieties of sugar cane associated with nitrogen fixation during decomposition were cultivated on Rapa Nui, as in Hawai'i, where the estimates originated (Lincoln and Vitousek, 2016).…”

“…What N is available in Polynesian agricultural systems that lack a symbiotic N fixer comes primarily from two sources: dissolved N in rainfall and from the fixation of nitrogen from the atmosphere by the microbial community as it decomposes sugarcane and other plants (Lincoln and Vitousek, 2016). The deposition rate of dissolved mineral N has been measured in some places and estimated for others across the globe, but the remote Pacific remains under studied in this regard.…”

“…Based on measurements in remote areas of Hawaii (Heath and Huebert, 1999;Raich et al, 2000), we estimate that deposition of dissolved N could be 0.0014 gN/m 2 /cm rain. For N fixation by microbes, we evaluate the lower half of the range of N fixation rates in decomposing sugar cane mulches in Polynesian cropping systems reported by Lincoln and Vitousek (2016), 4.8-20 kgN/ha/year.…”

Spatial patterns of seasonal crop production suggest coordination within and across dryland agricultural systems of Hawaiʻi Island

“…In situ field management that increases capture and storage of soil moisture, minimizes moisture loss, and extends periods of sufficient moisture would extend the growing season and increase staple root crop yields. Mulching with organic material was an important practice in all the systems for both moisture conservation and nutrient inputs (e.g., Lee et al 2006, Lincoln andVitousek 2016). Rock mulches, too, likely improved ʻuala yields (see Marshall et al 2017) by reducing evaporation of soil moisture (Allen 2004, Graf et al 2008) and improving soil nutrient availability, as is suggested from work in Rapa Nui .…”

Microbial biomass C and N dynamics, and 15N incorporation into microbial biomass under faba bean, canola, barley, and summer fal- low in a Gray Luvisol

“…Hence, relationship between microbial biomass in soil and effectiveness of management on agricultural production will aid in further understanding and possibly manipulation of nutrient flow on a terrestrial scale (McGill and Myers, 1987;Lincoln and Vitousek, 2016). Microbial biomass in agricultural soil is affectted by crop rotation (McGill et al, 1986), tillage (Carter and Rennie, 1982;Lynch and Panting, 1980a, b), soil texture (van Veen et al, 1987) and alternation of moist and dry conditions (Bottner, 1985).…”