alexa Influence of Nitrogen Limitation and Long-Term Use of Rockwool on Nitrous Oxide Emissions in Hydroponic Systems | OMICS International
ISSN: 2376-0354
Journal of Horticulture
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Influence of Nitrogen Limitation and Long-Term Use of Rockwool on Nitrous Oxide Emissions in Hydroponic Systems

Shin-nosuke Hashida1*, Kazuyoshi Kitazaki2, Kazuhiro Shoji1, Fumiyuki Goto1 and Toshihiro Yoshihara1
1Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 1646 Abiko, Chiba 270-1194, Japan
2National Agriculture and Food Research Organization (NARO), Kyushu Okinawa Agricultural Research Center, Japan
Corresponding Author : Shin-nosuke Hashida
PhD, Environmental Science Research Laboratory
Central Research Institute of Electric Power Industry (CRIEPI)
1646 Abiko, Chiba 270-1194, Japan
Tel: +81-4-7182-1181
E-mail: [email protected]
Received April 17, 2014; Accepted November 27, 2014; Published December 1, 2014
Citation: Hashida SN, Kitazaki K, Shoji K, Goto F, Yoshihara T (2014) Influence of Nitrogen Limitation and Long-Term Use of Rockwool on Nitrous Oxide Emissions in Hydroponic Systems. J Horticulture 1:113. doi:10.4172/2376-0354.1000113
Copyright: © 2014 Hashida SN, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

To mitigate Nitrous Oxide (N2O) emissions derived from Nitrogen (N) fertilizer of agroecosystems, establishment of best management protocols for cultivation is necessary. Hydroponic systems using rockwool have the potential to reduce N2O emissions; however, the effects of nutrient condition and retained N compounds in rockwool on N2O emissions remain unclear. The primary objective of our study was to understand the crucial factors behind emissions of N2O. Tomato cultivation with low levels of nutrient showed reduced growth and yield, but increased N2O emission. In contrast, growth and N2O emissions were increased by cultivation with normal levels of nutrient and used (1-y-old) rockwool containing excess N compounds from the previous year's cultivation. Though the long-term use of rockwool significantly enhanced seasonal N2O emission, the availability of N2O precursors NO3- and NH4+ did not clearly explain the variation in N2O fluxes during cultivation. Rather, environmental factors, such as relative water content of rockwool in the rhizosphere, were significantly correlated to N2O emissions during cultivation under various conditions. We conclude that environmental factors most strongly influence the fate of available environmental substrates remaining in rockwool, and thereby control N2O emissions.

Abstract
To mitigate Nitrous Oxide (N2O) emissions derived from Nitrogen (N) fertilizer of agroecosystems, establishment of best management protocols for cultivation is necessary. Hydroponic systems using rockwool have the potential to reduce N2O emissions; however, the effects of nutrient condition and retained N compounds in rockwool on N2O emissions remain unclear. The primary objective of our study was to understand the crucial factors behind emissions of N2O. Tomato cultivation with low levels of nutrient showed reduced growth and yield, but increased N2O emission. In contrast, growth and N2O emissions were increased by cultivation with normal levels of nutrient and used (1-y-old) rockwool containing excess N compounds from the previous year's cultivation. Though the long-term use of rockwool significantly enhanced seasonal N2O emission, the availability of N2O precursors NO3- and NH4+ did not clearly explain the variation in N2O fluxes during cultivation. Rather, environmental factors, such as relative water content of rockwool in the rhizosphere, were significantly correlated to N2O emissions during cultivation under various conditions. We conclude that environmental factors most strongly influence the fate of available environmental substrates remaining in rockwool, and thereby control N2O emissions.
Keywords
Denitrification; Greenhouse Gases (Ghgs); Nitrous Oxide (N2O); Rockwool; Tomato
Introduction
Increasing atmospheric Nitrous Oxide (N2O) is a major concern in agriculture because of its high Global Warming Potential (GWP) as a Greenhouse Gas (GHG); the GWP for a 100-year timescale of N2O is 310 times higher than that of Carbon Dioxide (CO2) [1]. In addition to its global warming potential, N2O is currently the largest ozone-depleting substance in the atmosphere and is projected to maintain that status throughout the 21st century [2]. It is recognized that agriculture is responsible for two-thirds of global N2O emissions, and that N2O formation is a result of excessive Nitrogen (N) fertilization [3]. Feasible ways to mitigate N2O emissions in agricultural production include improving nutrient use efficiency of crops, improving land-management and cultivation practices, and abandonment of cultivation [4]. Inorganic N compounds in soil, such as Nitrate (NO3-) and Ammonium (NH4+), that are not taken up by plants can be processed through the bacterial respiratory pathways of nitrification and/or denitrification, and converted to N2O [5-7]. A number of studies have reported the significant role of microbial respiration in N2O production in soil [8,9]. However, soil environmental factors, in addition to size and composition of the microbial community, also influence N2O fluxes [10,11].
Currently, the plant and horticultural industries are focused on stable production, quality improvement, and high yield. An increasing number of studies focusing on the environmental impacts of horticultural practices have reported that GHG emissions from horticultural production arise mainly from cultivation processes rather than from industrial production of materials used in cultivation (e.g., electricity, fertilizer, biocides, and rockwool) [12-15]. N2O emissions and total nitrogen losses from rockwool systems are predicted to be caused primarily by microbial denitrification or chemodenitrification [16,17]. However, the extent to which changes in N2O emissions are explained by changes in management practices during the cultivation process remains elusive. In our previous work, hydroponic cultivation using rockwool produced substantial tomato fruit yield while lowering CO2 emissions during the growing season, by suppressing microbial proliferation in the rhizosphere [18]. On the other hand, conventional hydroponics did not effectively mitigate N2O emissions. Consequently, emission of N2O was governed by neither the size nor the composition of the microbial community. Thus, as for soil-based cultivation [19], abiotic stimulation of rhizobacteria is important to N2O emissions from rockwool systems, although microbial proliferation is also essential for N2O production. Environmental factors can influence microbial and chemodenitrification [17], but the effects of nutrient dynamics, such as macro- and microelements, on N2O fluxes in the rhizosphere during crop cultivation remain unclear. A predictable factor for microbial stimulation in the rhizosphere is the concentration and form of inorganic N compounds. Enzymatic activity and availability of substrates for denitrification could act as determinants for N2O production. In addition to the concentration and form of inorganic N compounds, other elements are likely to affect the absorption of NO3- and NH4+ by plants, and thus influence the nutritional status of the rhizosphere. Understanding the precise mechanisms of N2O production in rockwool systems is important to controlling N2O emissions from these systems.
In this study, we measured GHG emissions and nutrient dynamics using hydroponics with a rockwool system. Our objective was to examine the effect of long-term use of rockwool, nutrient concentration, and the interaction between these factors on N2O flux during hydroponic cultivation. In addition, we explored the factors that are relevant to N2O emissions to better understand management practices for mitigating N2O emissions.
Materials and Methods
Plant material and growth conditions
Tomato (Solanum lycopersicum, Momotaro) plants were hydroponically grown with rockwool in an air-conditioned greenhouse in Abiko, Chiba, Japan (35.9°N, 140.0°E) from April to September 2012. The average temperature was set at 23 ± 3°C. Each three-week-old tomato seedling grown in rockwool block (Grotop Master, Gordan, Denmark) was transplanted to a 1/2000 a (12 L) Wagner pots filled with freshly prepared or once-used rockwool blocks sub-irrigated with a nutrient solution of the following composition (mg L-1) 179 N-NO3-, 18 N-NH4+, 92 P, 312 K, 177 Ca, 46 Mg, 2.1 Fe, 1.2 Mn and B, 0.07 Zn, 0.02 Cu and Mo (Otsuka House Solution A, Otsuka Chemical Co. Ltd., Osaka, Japan) with Electrical Conductivity (EC) of 1.0 dS m-1 (EC1.0) or 0.5 dS m-1 (EC0.5). During cultivation, the nutrient solution was supplied using 100 mL h-1 drip irrigation. Cultivation with freshly prepared rockwool and EC0.5 is malnutrition condition while cultivation with once-used rockwool and EC1.0 seems to be over-nutrition condition. Fruits were harvested every 2 wk from July to September, and total fresh weight per pot (fruit yield) and Brix value of individual harvested fruit were measured. Each plant height was measured at the end of cultivation. Old leaves and lateral shoots were removed on a weekly basis, dried, and weighed to estimate total shoot biomass. The investigations were performed with 3 replicates.
Experimental setup and gas sampling
We established four experimental blocks: Freshly Prepared Rockwool (FR) supplemented with nutrient solution at EC1.0 (FR-EC1.0) or EC0.5 (FR-EC0.5), and rockwool used for 1 y as FR-EC1.0 (UR) with nutrient solution at EC1.0 (UR-EC1.0) or EC0.5 (UR-EC0.5). CO2, CH4, and N2O emissions from the pots were captured using a closed-chamber technique as described before [18].
Measurements of GHG fluxes and ionic components
A gas-chromatography 7890A GC system equipped with a HaySepQ80/100 separation column (Agilent Tech. Inc., Santa Clara, CA, USA) was used to measure CO2, CH4, and N2O as described before [18]. Ionic components, Cl-, NO3-, PO4-, SO42-, Na+, NH4+, K+, Mg2+, and Ca2+, were extracted from rockwool of the rhizosphere with a homogenizer (Retsch MM300, Haan, Germany), followed by distilled water extraction, and measured with an ICS-1500 ion chromatography system (Dionex Corp., Osaka, Japan). Accuracy was established using cation mixed standard solution II and anion mixed standard IV (Kanto Chemical Co., Inc., Tokyo, Japan).
Statistical analysis
Two-way analysis of variance (ANOVA) with repeated measures was used to detect significant effects of rockwool type, nutrient concentration, and their interaction, using Excel 2008 Statistics for Windows (Microsoft Corporation, Redmond, WA, USA). Differences in growing-season plant growth, fruit yield, and N2O emissions among the four treatments were analyzed by Dunnette’s test using KyPlot 4.0 (KyensLab Incorporated, Tokyo, Japan). Correlations between N2O emissions, dynamics of nutrient concentrations, relative water content (RWC) were analyzed by Spearman’s rank correlation test using KyPlot 4.0.
Results and Discussion
Plant growth and fruit yield in conditions of over-nutrition and malnutrition
Shoot biomass at the end of the cultivation period was significantly higher in used rockwool (UR) treatments than in freshly prepared rockwool (FR) treatments regardless of nutrient concentration (Table 1).
However, the yield and Brix yield of plants grown in UR did not increase when nutrients were supplied at EC1.0. FR cultivation with diluted nutrients (FR-EC0.5) markedly lowered shoot biomass, yield, and Brix yield. This result strongly indicated that diluted nutrients were insufficient for normal fruit production. Compared to UR-EC1.0, UR-EC0.5 decreased shoot biomass but not yield or Brix yield, suggesting that nutrients retained in UR were utilized for fruit production, thereby enabling comparable yield to FR-EC1.0. UR-EC0.5 consequently demonstrated growth and fruit yield comparable to that of FR-EC1.0. As a result, a large amount of nutrients was supplied to UR-EC1.0 and achieved the highest shoot biomass. Thus, our cultivation conditions would represent over-nutrition (UR-EC1.0) and malnutrition (FR-EC0.5).
Distinct GHG emission characteristics in different cultivation systems
It is known that nitrogen fertilization increases soil uptake of CH4 [20], implying that over-nutrition decreased CH4 emission when tomato was hydroponically cultivated with rockwool. Unexpectedly, seasonal CH4 emissions (1.51 gm-2) were observed only in UR-EC1.0, whereas the other cultivation plots showed absorption (Table 2).
However, the differences among rockwool type, nutrient concentration, and their interaction were not statistically significant.
The UR system significantly increased CO2 emissions (p=0.016), with 3.82 kg m-2 emitted from UR-EC1.0 compared to 2.07 kg m-2 emitted from FR-EC1.0, and 2.34 and 1.52 kg m-2 emitted from UR-EC0.5 and FR-EC0.5 respectively (Table 2). Significant differences were also observed between the different nutrient concentrations, with EC1.0 emitting 1.4 to 1.6 times more CO2 than EC0.5 (p=0.044); however, there were no significant interactions between rockwool type and nutrient concentration. These results clearly suggest that microbial proliferation in rockwool during the previous growing season directly contributed to CO2 emissions.
The highest total seasonal N2O emission (2.24 gm-2) was observed in UR-EC1.0 (Table 2). Compared to FR-EC1.0 and UR-EC0.5, UR-EC1.0 released 5.3 and 2.2 times more N2O respectively; dilution of nutrient solution thus mitigated N2O emissions. Plant growth, total yield, and Brix yield in UR-EC1.0 were comparable to those of the FR-EC1.0 treatment (Table 1). Unexpectedly, the lowest nitrogen application (FR-EC0.5) emitted almost twice as much N2O as FR-EC1.0, although this difference was not statistically significant. As observed for plant growth and yield, FR-EC0.5 mimicked conditions of malnutrition. The nutrient concentration may have been too low to be absorbed by tomato roots, and the unabsorbed N compounds (NO3- and NH4+) may have elicited rhizospheric nitrification or denitrification [18]. As shown in Table 2, there was a significant interaction between rockwool type and nutrient concentration (p=0.047), so the factors inducing N2O emissions were complex.
On average, N loss as N2O amounted to 0.27 g out of 8.7 g (3.1%) in FR-EC1.0 (Table 3), comparable to levels reported by other researchers [21].
The fact that FR-EC0.5 lost 17.4% of N supplied in solution strongly suggested that unutilized rhizospheric N compounds could be readily transformed and released as N2O. Therefore, plants’ ability to take up N from rhizosphere could be crucial for the control and mitigation of N2O emissions. On the other hand, UR-EC1.0 released 1.4 g N as N2O, though actual nitrogen retained inside UR was enigmatic. UR at the beginning of cultivation contained greater amounts of NO3- and NH4+, precursors of N2O (Figure 1).
It is plausible that these nutrients were remnants from the previous season, and the rapid decrease in their concentrations coincided with N2O emission during the first four weeks (Figure 1, phase I). Therefore, at present we cannot claim whether the N released as N2O was derived from the nutrient inputs during the experiment, or from nutrients remaining from the previous season. Nevertheless, increased N2O emission from the UR system strongly suggested that excess N fertilizer residing in rockwool was transformed to N2O rather than providing a desirable effect of additional fertilizer for tomato plants as described above (Table 1).
Nutrient status and interactions in the rhizosphere
Over the course of the growing season, we found no statistically significant interactions in low-molecular-weight ionic components between rockwool type and nutrient treatment (Table 4).
Predictably, macro-elements (N-NO3-, P-PO4-, and K+) demonstrated significantly different fluctuations between nutrient concentrations.
Rockwool type also showed significance in NO3-, NH4+, PO4-, and K+, confirming that UR retained soluble forms of macro-elements unabsorbed during the last growing season. However, there were neither strong nor significant Spearman's rank correlations between N2O emission and PO4-, with the maximum coefficient (-0.39) at FR-EC1.0; between N2O emission and K+, the maximum coefficient (-0.25) was at FR-EC1.0. Despite the fact that NO3- and NH4+ are substrates for N2O production, neither demonstrated a prominent correlation to N2O emission (Table 5).
Even when data obtained from different cultivation periods were analyzed separately, no strong or significant correlations between N-nutrient concentrations and N2O emissions were observed (data not shown). These results strongly suggest that remnant N compounds do not act as determinants for N2O emission, although they undoubtedly increase the potential amount of N2O emission.
Absorptivity of nutrients is generally dependent on mutual interactions in the rhizosphere. For example, excessive amounts of redox-active microelements (e.g., iron, zinc, and copper) can cause root oxidative stress via generation of reactive oxygen species (ROS). Then, antioxidant defense systems that can protect cells from oxidative damage and scavenge harmful ROS consume reduced forms of nicotinamide adenine dinucleotide (NADH) and thereby decrease the amount of reductant power available to reduce NO3- and NO2- to NH4+ in root cells. Because the concentrations of intracellular NO3- and NH4+ are directly involved in feedback processes regulating their transport systems, the status of microelements in the rhizosphere may influence N2O emission by disturbing plants' absorption of NO3- and NH4+.
To explore underlying factors correlated with N2O emissions in rockwool cultivation (hydroponics), fluctuations in total macro- and microelements (potassium, calcium, sodium, phosphorus, magnesium, iron, zinc, manganese, copper, boron, aluminum) and relative water content (RWC) in FR and UR were investigated. Contrary to our expectations, there were no strong correlations between these microelements and N2O emissions (data not shown). However, we found significant and moderate correlations between %RWC and N2O emissions in the FR system and in UR-EC0.5 (Table 5). High %RWC generally indicates anaerobic conditions in the rhizosphere, allowing rhizospheric microbial denitrification to proceed [22]. Because denitrification is a respiratory process used as an alternative to oxygen respiration under low oxygen or anoxic conditions [23], a steady state characterized by higher water content in the rhizosphere may cause anoxic conditions and trigger denitrification processes in rockwool. An exception was the UR-EC1.0 treatment, which demonstrated no correlation to %RWC, as %RWC is just one of numerous triggering factors or antecedents to the onset of N2O emissions.
Conclusion
Our findings have demonstrated that a considerable range of remnant nutrients, such as NO3- and NH4+, in the rhizosphere in hydroponic rockwool systems, are insufficient to promote N2O emission, despite their importance to N2O production. Our result provided a possibility that an anaerobic condition in rhizosphere could control N2O production via denitrification pathway in the presence of remnant N-nutrient. So, an aerobic condition may be undesirable for mitigating N2O emission, suggesting the importance of management of drainage in rockwool. We conclude that environmental factors dominate the fates of available environmental substrates retained in rockwool, thereby controlling N2O fluxes. Our study does not exclude other rhizospheric environmental factors (e.g., temperature, pH, EC, dissolved oxygen, and oxidation-reduction potential), which were not assessed in this study. It is challenging but necessary to determine the most influential factors in N2O emissions and to establish cultivation-management protocols to mitigate these emissions. Simultaneous real-time monitoring of N2O fluxes and of environmental factors that affect these fluxes will assist in progressing toward this goal.
Acknowledgments
We thank Mr. Hiroshi Shimura, Ms. Miki Ueda, and Ms. Mari Sato (Co. Ltd. Ceres) for their technical assistance in the cultivation, sampling and gas analyses, and helpful comments on the manuscript. This work was supported by Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research to T. Y., F. G., K. S. and S.-n.H., Grant Number 22380139.
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