Biogeochemical indicators of elevated nitrogen deposition in semiarid Mediterranean ecosystems

Nitrogen (N) deposition has doubled the natural N inputs received by ecosystems through biological N fixation and is currently a global problem that is affecting the Mediterranean regions. We evaluated the existing relationships between increased atmospheric N deposition and biogeochemical indicators related to soil chemical factors and cryptogam species across semiarid central, southern, and eastern Spain. The cryptogam species studied were the biocrust-forming species Pleurochaete squarrosa (moss) and Cladonia foliacea (lichen). Sampling sites were chosen in Quercus coccifera (kermes oak) shrublands and Pinus halepensis (Aleppo pine) forests to cover a range of inorganic N deposition representative of the levels found in the Iberian Peninsula (between 4.4 and 8.1 kg N ha(-1) year(-1)). We extended the ambient N deposition gradient by including experimental plots to which N had been added for 3 years at rates of 10, 20, and 50 kg N ha(-1) year(-1). Overall, N deposition (extant plus simulated) increased soil inorganic N availability and caused soil acidification. Nitrogen deposition increased phosphomonoesterase (PME) enzyme activity and PME/nitrate reductase (NR) ratio in both species, whereas the NR activity was reduced only in the moss. Responses of PME and NR activities were attributed to an induced N to phosphorus imbalance and to N saturation, respectively. When only considering the ambient N deposition, soil organic C and N contents were positively related to N deposition, a response driven by pine forests. The PME/NR ratios of the moss were better predictors of N deposition rates than PME or NR activities alone in shrublands, whereas no correlation between N deposition and the lichen physiology was observed. We conclude that integrative physiological measurements, such as PME/NR ratios, measured on sensitive species such as P. squarrosa, can provide useful data for national-scale biomonitoring programs, whereas soil acidification and soil C and N storage could be useful as additional corroborating ecosystem indicators of chronic N pollution.

Risk of nitrate pollution in agricultural systems

Se propone una metodología que nos permita evaluar un óptimo manejo de la fertirrigación integrando aspectos agronómicos y medioambientales.

In situ conversion of protochlorophyllide b to protochlorophyllide a in barley. Evidence for a novel role of 7-formyl reductase in the prolamellar body of etioplasts

We recently put forth a model of a protochlorophyllide (Pchlide) light-harvesting complex operative during angiosperm seedling de-etiolation (Reinbothe, C., Lebedev, N., and Reinbothe, S. (1999) Nature 397, 80–84). This model, which was based on in vitro reconstitution experiments with zinc analogs of Pchlide a and Pchlide b and the two NADPH:protochlorophyllide oxidoreductases (PORs), PORA and PORB, of barley, predicted a 5-fold excess of Pchlide b, relative to Pchlide a, in the prolamellar body of etioplasts. Recent work (Scheumann, V., Klement, H., Helfrich, M., Oster, U., Schoch, S., and Rüdiger, W. (1999) FEBS Lett. 445, 445–448), however, contradicted this model and reported that Pchlide b would not be present in etiolated plants. Here we demonstrate that Pchlide b is an abundant pigment in barley etioplasts but is rather metabolically unstable. It is rapidly converted to Pchlide a by virtue of 7-formyl reductase activity, an enzyme that had previously been implicated in the chlorophyll (Chl) b to Chl a reaction cycle. Our findings suggest that etiolated plants make use of 7-formyl reductase to fine tune the levels of Pchlide b and Pchlidea and thereby may regulate the steady-state level of light-harvesting POR-Pchlide comple

The role of cover crops in irrigated systems: water balance, nitrate leaching and soil mineral nitrogen accumulation

Soil salinity and salt leaching are a risk for sustainable agricultural production in many irrigated areas. This study was conducted over 3.5 years to determine how replacing the usual winter fallow with a cover crop (CC) affects soil salt accumulation and salt leaching in irrigated systems. Treatments studied during the period between summer crops were: barley (Hordeum vulgare L.), vetch (Vicia villosa L.) and fallow. Soil water content was monitored daily to a depth of 1.3 m and used with the numerical model WAVE to calculate drainage. Electrical conductivity (EC) was measured in soil solutions periodically, and in the soil saturated paste extracts before sowing CC and maize. Salt leaching was calculated multiplying drainage by total dissolved salts in the soil solution, and use to obtain a salt balance. Total salt leaching over the four winter fallow periods was 26 Mg ha−1, whereas less than 18 Mg ha−1 in the presence of a CC. Periods of salt gain occurred more often in the CC than in the fallow. By the end of the experiment, net salt losses occurred in all treatments, owing to occasional periods of heavy rainfall. The CC were more prone than the fallow to reduce soil salt accumulation during the early growth stages of the subsequent cash crop.

Meta-analysis of strategies to control nitrate leaching in irrigated agricultural systems and their effects on crop yield.

Nitrate leaching (NL) is an important N loss process in irrigated agriculture that imposes a cost on the farmer and the environment. A meta-analysis of published experimental results from agricultural irrigated systems was conducted to identify those strategies that have proven effective at reducing NL and to quantify the scale of reduction that can be achieved. Forty-four scientific articles were identified which investigated four main strategies (water and fertilizer management, use of cover crops and fertilizer technology) creating a database with 279 observations on NL and 166 on crop yield. Management practices that adjust water application to crop needs reduced NL by a mean of 80% without a reduction in crop yield. Improved fertilizer management reduced NL by 40%, and the best relationship between yield and NL was obtained when applying the recommended fertilizer rate. Replacing a fallow with a non-legume cover crop reduced NL by 50% while using a legume did not have any effect on NL. Improved fertilizer technology also decreased NL but was the least effective of the selected strategies. The risk of nitrate leaching from irrigated systems is high, but optimum management practices may mitigate this risk and maintain crop yields while enhancing environmental sustainability.

Nitrate-induced early transcriptional changes during imbibition in non-after-ripened Sisymbrium officinale seeds.

We have here demonstrated for the first time that nitrate not only accelerates testa rupture of non- AR seeds but also modifies expression pattern of the cell-wall remodeling proteins (mannanases; SoMAN6 and SoMAN7) and key genes belonging to metabolism and signaling of ABA (SoNCED6, SoNCED9, SoCYP707A2 and SoABI5) and GAs (SoGA3ox, SoGA20ox, SoGA2ox and SoRGL2). These results were obtained during Sisymbrium officinale seed imbibition in the absence of endosperm rupture. Exogenous ABA induced a notable inhibition of testa rupture in both absence and presence of nitrate being this effect sharply reversed by GA4+7. However, nitrate was capable to provoke testa rupture in absence of ABA synthesis. The expression of SoMAN6 and SoMAN7 were positively altered by nitrate. Although ABA synthesis seems apparent at the start of non-AR seed imbibition, taken together the results of SoNCED6, SoNCED9 and SoCYP707A2 expression seem to suggest that nitrate leads to a strong net ABA decrease. Likewise, nitrate positively affected the SoABI5 expression when the SoNCED9 expression was also stimulated. By contrast, at the early and final of imbibition, nitrate clearly inhibited the SoABI5 expression. The expression of SoGA2ox6 and SoGA3ox2 are strongly inhibited by nitrate whereas of SoGA20ox6 was stimulated. On the other hand, SoRGL2 transcript level decreased in the presence of nitrate. Taken together, the results presented here suggest that the nitrate signaling is already operative during the non-AR S. officinale seeds imbibition. The nitrate, in cross-talk with the AR network likely increases the favorable molecular conditions that trigger germination.

The nitrate-afterripening crosstalk is involved in the testa rupture of sisymbryum offinales seed

The loss of seed dormancy can occur by exposing the seed at low moisture storage conditions (afterripening; AR). Since a positive GA:ABA ratio play a key role in the reactivation of germination of non-dormant seeds, it seems obvious that a remarkable effect of AR is the decreasing of both ABA levels and sensitivity, as well as the increment of GA synthesis and sensitivity. ABA levels are regulated by control both of its biosynthesis thorough the 9-cis-epoxycarotenoid dioxygenase (NCED) encoding genes and its catabolism mediated mainly by ABA-8¿-hydroxylases (CYP707A). On the other hand, the last steps of the GA biosynthesis pathway should be involved to control its levels. Namely, GA20ox and GA3ox catalyzing the biosynthesis of active GA and GA2ox which catalyzes the GA inactivation. The presence of nitrate accelerates the sensu stricto germination of non-AR S. officinale seeds. Here, we demonstrate that in AR seeds nitrate also alters the expression pattern of key genes involved in ABA and GA metabolism and signalling (i.e. SoNCED6, SoNCED9, SoCYP707A2, SoABI5, SoGA3ox2, SoGA20ox6, SoGA2ox6 and SoRGL2). These results suggest that the nitrate signalling is also operative during imbibition of AR S. officinale seeds.

Cover crops effect on farm benefits and nitrate leaching: Linking economic and environmental analysis.

Introducing cover crops (CC) interspersed with intensively fertilized crops in rotation has the potential to reduce nitrate leaching. This paper evaluates various strategies involving CC between maize and compares the economic and environmental results with respect to a typical maize?fallow rotation. The comparison is performed through stochastic (Monte-Carlo) simulation models of farms? profits using probability distribution functions (pdfs) of yield and N fertilizer saving fitted with data collected from various field trials and pdfs of crop prices and the cost of fertilizer fitted from statistical sources. Stochastic dominance relationships are obtained to rank the most profitable strategies from a farm financial perspective. A two-criterion comparison scheme is proposed to rank alternative strategies based on farm profit and nitrate leaching levels, taking the baseline scenario as the maize?fallow rotation. The results show that when CC biomass is sold as forage instead of keeping it in the soil, greater profit and less leaching of nitrates are achieved than in the baseline scenario. While the fertilizer saving will be lower if CC is sold than if it is kept in the soil, the revenue obtained from the sale of the CC compensates for the reduced fertilizer savings. The results show that CC would perhaps provide a double dividend of greater profit and reduced nitrate leaching in intensive irrigated cropping systems in Mediterranean regions.