Carbon sink reduction by fruit removal triggers respiration but not nitrous oxide emissions from the root zone of cucumber

Abstract

Root exudation of organic carbon (C) is generally believed to be the cause of positive effects of root activity on nitrous oxide (N 2 O) emissions. We tested the effects of root exudation in an actual soil-plant system on N 2 O emissions while excluding most other potential factors. The C source/sink ratio in cucumber was changed by removing fruits to increase root exudation. Root-zone emissions of carbon dioxide (CO 2 ) and N 2 O were monitored in complete stands of adult plants in a greenhouse.Whereas CO 2 emissions rapidly increased as a result of fruit removal the N 2 O emissions were completely unaffected. After cutting the shoots CO 2 emissions decreased within 2 weeks in both the fruit removal treatment and the control to a value significantly lower than that before the start of the treatments. However, N 2 O emissions immediately exhibited a short peak, which was significantly higher in the fruit removal treatment compared to the control. Thereafter N 2 O emissions in both treatments remained on the same level but considerably higher than before shoot cutting.We concluded that in a well-aerated root zone, a root exudation pulse does not necessarily increase N 2 O emissions, because C substrates are quickly respired by microorganisms before they can support heterotrophic denitrification. The results further indicate the significance of dying/dead roots for the creation of denitrificaton hotspots, which likely result from providing C substrates as well as poorly aerated habitats. K E Y W O R D Sanaerobic microorganisms, Cucumis sativus L., denitrification hot-spots, dry matter, root exudation

Evaluating recycling fertilizers for tomato cultivation in hydroponics, and their impact on greenhouse gas emissions

“…In contrast, daily N 2 O fluxes for NPK, CRO, and AUR ranged between 0.14 and 0.25 g N 2 O-N and were barely detectable with the used methodology. Only the total cumulative emissions of S+V (1335 ± 697 g N 2 O-N ha −1 ) were close to published findings for N 2 O emissions in greenhouse hydroponic cultivation (Daum 1996 ; Daum and Schenk 1996 ; Nett et al 2019 ). Despite the higher N 2 O emissions associated with the S+V treatment, daily emissions appear to be, on average, below values found in the literature.…”

“…As described by (Nett et al 2019), shoot and fruit removal can be contributing factors in increased root zone respiration, which is probably due to an increased release of root exudates. However, despite a specific increase in CO 2 prior to the removal of fruits, the authors did not observe a change in N 2 O production at the harvest phase.…”

Seasonal Nitrous Oxide Emissions From Hydroponic Tomato and Cucumber Cultivation in a Commercial Greenhouse Company

“…The study of Nett et al (2019) also showed that N 2 O emission rates can strongly increase if sufficient organic C is available in the substrate, as demonstrated by a peak of N 2 O emissions following the degradation of roots after cutting shoots. The N 2 O emissions from hydroponic systems are probably mainly due to denitrification, as nitrate is typically used as primary N-fertilizer in such systems (de Kreij et al, 2003).…”

“…Llorach-Massana et al (2017) found that lettuce crops on perlite bags emitted 0.7-0.9% of the applied N-fertilizer as N 2 O. Similarly, low N 2 O emission rates were found by (Nett et al, 2019) for cucumber cultivation on substrate-filled pots.…”

“…These studies from only two teams, however, were selective and conducted under laboratory experimental greenhouse conditions and may not properly reflect N 2 O emissions from commercial tomato and cucumber production. A more recent study on cucumbers cultivated in a large phytotron with cabins of 30 m², which was specifically constructed to measure gas fluxes in the root zones of multiple plants, found relatively low N 2 O emission rates of ∼17 g N 2 O-N ha −1 d −1 during plant cultivation (Nett et al, 2019). Nevertheless, the same study also showed that under certain conditions, like the enhanced degradation of roots after fruit removal and shoot-cutting, N 2 O emission rates can rise to high levels in the range of 180-390 g N 2 O-N ha −1 d −1 .…”