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.
Field drainage and nitrogen leaching: some experimental results
Public concern continues to be expressed about the quality of water discharged from agricultural lands. High concentrations of nitrate nitrogen are often blamed on modern agricultural practice in general and to land drainage in particular (Royal Commission on Environmental Pollution, 1979, para. 4.38). However, undrained land also discharges water by flow within the soil and across the soil surface and it may discharge significant quantities of solutes.
Potentials of nitrification inhibitors in modern N‐fertilizer management
Modern agricultural practices require a new concept of N-fertilizer management in order to optimize N-utilization and avoid N-losses. Nitrification inhibitors or „N-stabilizers” fit very good into this conception. 
 
 
 
Dicyandiamide (DCD) is an efficient nitrification inhibitor and blocks the first step of nitrification for 1–3 months (depending on temperature). This effect is bacteriostatic (not bactericidal) and does not affect other (esp. C-heterotrophic) soil microorganisms („biological activity”). 
 
 
 
DCD is a non-toxic, water soluble compound and will be degraded to CO2, NH3 and H2O without any residues. 
 
 
 
There are various possibilities to use DCD: addition to liquid manure temporarily prevents oxidation of ammonium nitrogen e.g. of slurry or waste water from potato starch production. 
 
 
 
In combination with inorganic fertilizers like ammonium sulfate or urea (with 10% of total-N) it enables the farmer to control NH4-supply to crop plants in certain stages of growth and to gain certain operational advantages by less frequently split applications of N especially on sand and rendzina soils. 
 
 
 
Thus, the systematic use of nitrification inhibitors not only represents a progress in agricultural technique but also helps to substantially reduce risks concerning pollution of surface and ground waters that are sometimes inevitable consequences of agricultural production. 
 
 
 
Moglichkeiten von Nitrifikationshemmstoffen in der modernen Stickstoffdungung Moderne Landbewirtschaftungsmethoden erfordern ein neues Dungungskonzept mit dem Ziel, die Stickstoffausnutzung zu optimieren und N-Verluste zu verhindern. Nitrifikationshemmstoffe oder „N-Stabilisatoren” passen gut in dieses Konzept. Dicyandiamid (DCD) ist ein wirksamer Nitrifikationshemmstoff; er blockiert den ersten Schritt der Nitrifikation fur die Dauer von 1–3 Monaten (je nach Temperatur). Dieser Effekt ist bakteriostatisch (nicht bakterizid) und beeintrachtigt nicht andere (vor allem C-heterotrophe) Bodenorganismen („biologische Aktivitat”). DCD ist eine nicht toxische, wasserlosliche Verbindung, die letztlich zu CO2, NH3 und H2O abgebaut wird ohne Ruckstande. 
 
 
 
Es gibt verschiedene Moglichkeiten fur den Einsatz von DCD: 
 
 
 
Gulle oder Abwasser aus der Kartoffelstarkeproduktion („Kartoffelfruchtwasser”) zugesetzt, verhindert es die Oxidation des Ammoniumstickstoffs fur eine gewisse Zeit. 
 
 
 
In Kombination mit anorganischen Dungemitteln wie Ammoniumsulfat oder Harnstoff (mit 10% vom Ges.-N) ist es moglich, die NH4-N-Anlieferung zu Kulturpflanzen in bestimmten Entwicklungsabschnitten zu regulieren. Damit verbunden sind gewisse arbeitswirtschaftliche Vorteile gegenuber ublichen mehreren Teilgaben mit mineralischem Stickstoff vor allem auf Sanden und Rendzina-Boden. 
 
 
 
Die Anwendung von Nitrifikationshemmstoffen stellt somit nicht nur einen Fortschritt dar in agrotechnischer Hinsicht, sondern tragt auch wesentlich dazu bei, mogliche Risiken einer Belastung von Oberflachen- und Grundwasser mit Nitrat zu verringern, die manchmal unvermeidbare Folgen der landwirtschaftlichen Produktion darstellen konnen.