Vinyl-film mulch: a practice for sweet potato (Ipomoea Batatas Lam. var. Edulis Makino) cultivation to reduce nitrate leaching
Abstract Under conventional cultivation practices in Southern Kyushu, Japan, a considerable amount of nitrate-N is leached from agricultural fields into ground water. The leaching processes are accelerated by high permeability of the volcanic ash soil, high rainfall and high summer temperatures. The present work was instigated to study the possibilities of controlling nitrate-N leaching by regulating the infiltration of rain water in the soil, the source of percolation below the root zone. Nitrate-N leaching from the root zone of sweet potatoes to deeper soil layers was compared in three treatments: maximum soil surface mulching with 0.03 mm vinyl-film with holes at ridge tops to supply plants with enough soil moisture to meet the crop water requirement (maximum vinly-film mulch, MVM); applications of fertilizer containing a nitrification inhibitor dicyaniamide (DCD); and conventional cultivation practices. NPK compound fertilizer (N 8 P 5.2 K 16.6 ) containing the ammonium form of nitrogen 900 kg·ha −1 and PK compound fertilizer P 8.7 K 16.6 600 kg·ha −1 were applied in the MVM and conventional plots. DCD-fertilizer (N 15 P 6.5 K 12.5 ), maintaining the same rate of absolute N with adjustment of P and K using fused magnesium phosphate (P 8.6 ) and potassium chloride (K 41.5 ), were applied to the DCD plots. MVM increased surface run-off, resulting in least percolation among the treatments. The highest nitrate-N concentration, taken as an average for 2 years, in soil-water periodically sampled throughout the growing period at 210 cm depth below surface, was found 1.8 ppm in MVM, whereas in DCD and in conventional treatments the same was measured as 8.7 and 5.2 ppm, respectively, indicating the lowest level of nitrate leaching to be in the MVM treatment. The highest availability of mineral nitrogen in the root zone of the MVM treatment was confirmed by the highest plant-top-uptake of nitrogen, Leaf Area Index and vine growth. DCD fertilizer did not contribute to the control nitrate-N leaching. There was no significant difference in root yields among the three treatments.
Potato-Cabbage Double Cropping Effect on Nitrate Leaching and Resource-Use Efficiencies in an Irrigated Area
Abstract To reduce the nitrate leaching risk after potato ( Solanum tuberosum L.) harvest and improve nitrogen fertilizer-use efficiency, a potato-cabbage double cropping system (DCS) was established at Hetao, North China, an arid area with irrigated land. A two-year field experiment demonstrated that planting early-maturing potato cultivar under plastic mulch shortened its growth period by 14 d and allowed a second crop of cabbage to scavenge the soil residual NO 3 − S-N to a depth of 160 cm, substantially reducing the risk of nitrate leaching into groundwater. The yearly total N uptake in DCS was about 110 kg ha −1 more than that in the conventional cropping system (CCS), i.e., mono potato planting. This accounted for apparent nitrogen recovery (ANR) improvement of 16.90%–26.57% in the DCS as compared to that in the CCS for both years. As a result, the soil residual NO 3 − -N in the 0–160 cm soil profile in the DCS was lower than that in the CCS. The solar energy-use efficiency and soil-use efficiency were also substantially increased with DCS.
Stickstoffbilanz von 15N-Harnstoff bzw. 15N-Ammonsulfatsalpeter mit Dicyandiamid in Gefäßversuchen zu Grünhafer und Sommerweizen
In Gefasversuchen mit Grunhafer und Sommerweizen wurde auf der Basis gleicher N-Mengen die Wirkung von 15N-Harnstoff (Ha) und 15N-Ammonsulfatsalpeter (ASS) unter Zugabe des nicht markierten Nitrifikationshemmstoffes Dicyandiamid (DCD, und Anrechnung dessen N-Gehaltes) auf Ertrage, N-Entzuge, Verteilung des 15N-Stickstoffs in Pflanzen und Boden (sandiger Lehm, pH 6.5) sowie Verbleib des DCD untersucht mit folgendem Ergebnis: 
 
1 
Im Neubauerversuch mit Grunhafer wurden 62–69% des Dunger-N im Spros, 21–31% in den Wurzeln und 4–9% im Boden gefunden. Im Mitscherlichversuch mit Sommerweizen wurden 85–88% des Dunger-N in Kornern und Stroh, 7–10% in den Wurzeln und 3–4% im Boden gefunden. Die 15N-Recovery-Quote lag in allen Fallen zwischen 98 und 100% und bestatigte damit ein praktisch verlustfreies Arbeiten. 
 
2 
Die Ertrage von Grunhafer (Spros) bzw. Sommerweizen (Korn und Stroh) lagen in den DCD-Varianten um 4–11% niedriger; dagegen waren die Stickstoffgehalte und -entzuge meist etwas hoher als in den DCD-freien Kontrollen. 
 
3 
Das wasserlosliche DCD wurde unter den Bedingungen dieser Gefasversuche (starke Durchwurzelung, gunstige Temperatur, zweimaliges Angebot im fruhen Wachstumsstadium, langsamer bodenspezifischer Abbau), teilweise auch von den Pflanzen aufgenommen und vorwiegend in Blattern bzw. Stroh nachgewiesen. In Sommerweizenkornern waren die DCD-Gehalte dagegen sehr gering (maximal 3 ppm DCD-N), in den Wurzeln nicht nachweisbar. Vergleichende Untersuchungen im Erntematerial von Feldversuchen ergaben kein DCD in Kornern und nur sehr wenige ppm DCD im Stroh. 
 
4 
In den DCD-Varianten ist der im Boden verbliebene Rest-N hoher, vermutlich aufgrund der langeren Einwirkungszeit des Ammonium auf die organische Substanz des Bodens. 
 
 
 
 
 
Recovery of Nitrogen of 15N-urea and 15N-ammonium sulfa-nitrate with addition of dicyandiamide in pot trials with green oats and spring wheat 
 
 
 
In pot trials with green oats and spring wheat, the effect of 15N-urea (UR) and 15N-ammonium sulfa-nitrate (ASN) with addition of the unlabelled nitrification inhibitor dicyandiamide (DCD) was investigated on the basis of equal amounts of N by fully taking into account the DCD-N, with respect to yield, N-removal, distribution of 15N in plants and soil (sandy loam, pH 6.5) as well as the fate of DCD. 
 
1 
In Neubauer experiments with green oats, 62–69% of fertilizer-N were found in the shoots, 21–31% in the roots and 4–9% in the soil. In Mitscherlich trials with spring wheat, 85–88% of fertilizer-N were found in grain and straw, 7–10% in the roots and 3–4% in the soil. 15N-recovery, in all cases was between 98-100%, thus exhibiting pratically no loss in course of the experiment. 
 
2 
Yields of green oats (shoots), resp. spring wheat (grain and straw) were about 4-11 % lower in pots with DCD; on the contrary, N-contents and removal were somewhat higher compared to control without DCD. 
 
3 
With conditions given by these pot trials (strong root penetration, favourable temperature, repeated supply during early growth, slow soil-specific decomposition), water-soluble DCD was taken up partly by the plants and appeared mainly in leaves resp. straw. In grains of spring wheat, DCD-contents were, however, very low (maximum 3 ppm DCD-N); in roots it could not be detected. In comparison, analysis of crop-material in field trials showed no DCD in grains and only very few ppm DCD in straw. 
 
4 
In pots with DCD, the residual-N in the soil was higher, probably because of the prolonged time of reaction between ammonium and the organic matter of the soil.